Pag/pinag: I am guessing from examples I have seen and heard th

@Diegocorry I'm not sure I understand your question. Is that like "mag-aral" and "pag-aralin"? Can you give an example so we can address your question more accurately? Thanks!

Opo.
(1) Sinusulat ko ang email ngayon = I am writing an email now.
(2) Pinag-aaralan ko ang musika ngayon. = I am studying music now.
If I apply the "logic" of (1) to (2) I might say "Ainaralan ko ...." or maybe "Inaaralan ko ...."
I'm guessing that this is awkward because "aral" has an initial vowel, so "pinag" is used. So the question is: is it a general rule that if a "mag" verb has an initial vowel then pag/pinag is used to form the object focus? Or, if this is not the case, what determines the use of pag/pinag?
I hope this clarifies the question.
Salamat po!

@Diegocorry Before I answer your question, let me just make some adjustments on your 2 sentences to make them more correct.For your sentence 1, the infinitive of the verb is “isulat”. “Sulatin” is not a proper verb.Isulat (to write something; object-focus (o-f)): isinulat, isinusulat, isusulat.
- Some of us have a tendency to drop the first “i” for the past and present tenses, hence, you might encounter “sinulat” and “sinusulat” (the one you used). However, that “i” cannot be removed for the future tense, lest we come up with the future tense of another verb form, “sumulat”.Sumulat (to write; subject-focus(s-f)): sumulat, sumusulat, susulat.It is acceptable to use “sinusulat” as you did in your sentence, but you should remember that strictly speaking, that should be “isinusulat”.“Pinag-aaralan ko ANG musika ngayon” translates to “I am studying THE music now”. If what you meant was “music” as a field of study, the “ang” should not be there and it would be more natural to state the sentence as “Musika ang pinag-aaralan ko ngayon”. It is not likely that we’d say that as “Pinag-aaralan ko Musika ngayon” because the syntax sounds a bit off.Now, to answer your question.The affixes used to form verbs have nothing to do with what letter a root word starts with. There is also no rule that make a set of affixes apply to a set of root words. Instead, every root word should be considered as independent from other root words with regard to the affix or affixes it may sensibly be paired with.With “aral”, for example, its most common verb forms are:MAG-aral (s-f): NAG-aral, NAG-aaral, MAG-aaral
Nag-aral si Mary ng Musika. = Mary studied Music.PAG-aralan (o-f): PINAG-aralan, PINAG-aaralan, PAG-aaralan
Musika ang pinag-aralan ni Mary. = Mary studied/took up Music.
- Your “inaaralan ko” should use this verb form instead - “pinag-aaralan”.PAG-aralin (o-f): PINAG-aral, PINAG-aaral, PAG-aaralin
Pinag-aral si Mary ng Musika ng tatay niya. = Mary’s father made her take up Music.aralIN (o-f): INaral, INaaral, aaralIN (not very often used, but may be a substitute for “PAG-aralan”
Musika ang inaral ni Mary. = Mary studied/took up Music.Note that even if “um” is a very common s-f affix, there is no “umaral” verb.Here are the most common verb forms for “sulat’:sUMulat (s-f): sUMulat, sUMusulat, susulat
Sumulat si Mary sa akin. = Mary wrote to me.MAGsulat (s-f): NAGsulat, NAGsusulat, MAGsusulat
Nagsulat si Mary ng liham para sa akin. = Mary wrote a letter for me.Isulat (o-f): IsINulat, IsINusulat, Isusulat
Isinulat/Sinulat ni Mary sa akin ang problema niya sa iyo. = Mary wrote to me her problem with you.sulatAN (o-f): sINulatAN, sINusulatAN, susulatAN
Sinulatan mo na ba si Mary? = Have you already written to Mary?PAsulatIN (o-f): PINAsulat, PINAsusulat, PasusulatIN
Pinasulat ko si Mary sa iyo. = I told/asked Mary to write to you.MAsulatAN (o-f): NAsulatAN, NAsusulatAN, MasusulatAN - may also at times be used in place of “sulatan”.
Nasulatan mo na ba si Mary? = Have you already written to Mary?
Sorry, nasulatan ko ang libro mo. Akala ko kasi libro ko siya. = I am sorry for accidentally writing on your book. It’s because I thought it was my book.

@Tagamanila Thank you so much! It answers my question perfectly, and it's a better lesson than I could get from any book or from Rosetta Stone! You did raise one question that I hope you don't mind my asking: If "sulat" can be used in MAG, UM or MA forms, are there different shades of meaning associated with each form? For example, are the following three sentences understood the same, or do they have different nuances?
1. Nagsulat si Mary ng liham.
2. Sumulat si Mary ng liham.
3. Nasulat si Mary ng liham.
Thank you!

@Tagamanila I see in many of your very informative replies that you quite often mention a root word's "most common verb forms" or conjugations. Is there a way for us newbies to find out or to recognize or to latch on to "only" the most common forms initially and to learn others at a later date ?

@stevesmi My calling them “most common verb forms” is quite subjective. Maybe the better description for them would be “the verb forms that come to mind”. However, I suppose I get to think about them because they are commonly used. All the verb forms used in the Example Sentences on this site are common forms since their selection was based on the frequencies they were found in Filipino documents.As far as the conjugations of the verbs are concerned, their patterns are almost all the same that it should not be too difficult to remember them. Don’t worry about using the wrong affix or affixes because when you are talking to a native speaker, we would be able to figure out the appropriate verb form anyway. What is more important is that you are using the correct root word.Our own President is guilty of using the wrong affixes often. He’s from Davao in Mindanao and Tagalog is not their native language there. He tends to use the “mag” prefix when “um” or something else is the correct one. Haha. Just the same, we understand what he’s saying, except when he throws in some Visayan words in there, too.

@Diegocorry You're welcome!For the “ma” prefix that I gave in the example, it is actually the “ma + an” affix combination and not just “ma”. In some Tagalog regions, they do use only the “ma” prefix with “sulat” such that “Nasulat si Mary ng liham” will be correct. I think, although I’m not absolutely sure, that to them it is a statement in the present tense and is, therefore, the equivalent of “Nagsusulat si Mary ng liham” or “Sumusulat si Mary ng liham” (Mary is writing a letter). As you used them in your sentences 1 and 2, “magsulat” and “sumulat” would mean the same thing. They would, therefore, be interchangeable as they usually are. Both are also subject-focus verbs. “Nagsulat/Sumulat si Mary ng liham” = Mary wrote a letter.However, “MAsulatAN” is an object-focus verb and “Nasulatan si Mary ng liham” translates instead to “Mary was written a letter”. However, the sentence by itself will not make much sense unless the actor (the writer of the letter) is already understood.There are instances though when “magsulat” and “sumulat” may not mean exactly the same thing. If we have a sentence that says “Mary wrote a letter to the mayor”, we may say that as “Nagsulat/Sumulat si Mary ng liham sa mayor”. With “magsulat”, imagine Mary holding a pen and writing the letter or maybe typing it. With “sumulat”, imagine Mary sending a letter. It would not really matter either way in that sentence because they share the same goal of “writing to the mayor”. But strictly speaking, “magsulat” is about the act of writing itself while “sumulat” is usually understood to include the transmittal of the letter.If, let’s say, we have a pen that we have not used for a long time and we’re now wondering if it still works. We can ask the question, “Does this still write?” To express that in Filipino, we would only say it as “Sumusulat pa ba ito?”. We won’t say “Nagsusulat pa ba ito?” because “magsulat” is the act of writing and the pen can’t do that by itself. “Sumulat” works in this case because the focus is on the goal, the written output.Nagsúsulát ako nang namatáy ang ilaw. = I was writing when the lights went out. - The speaker is saying that the lights went out while he is actively writing something. The element of time, i.e., exact concurrence of the 2 events, is given importance here.Sumusulat ako sa iyo nang namatáy and ilaw. = I was writing to you when the lights went out. - Time is not given much importance here. The message the speaker was conveying was that in the middle of writing a letter to the person he’s talking to, the lights went out. He is not necessarily saying that he was actually writing at that very moment. He could have been doing something else, but within the time frame of writing the letter.

@Tagamanila Once more, a very instructive lesson. And once more, thank you very much!

@Tagamanila I would like to ask if I may copy and paste your replies so that I have a study guide for the questions I've asked.