@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.