@JohnD
Some verbs just have a “pag-“ prefix, but I’m not sure it has any special/significant meaning...it’s just a verb. Other examples of verbs with a pag- prefix: pag-aralan (to study something), pag-usapan (to discuss something), pag-sabihan (to scold/reprimand someone). “Pagsamahin” would be “to combine/join something.”
The root here would be “sama”. “kasama” that you mentioned earlier is actually made by a prefix “ka-“ and the root “sama”. The “ka” prefix means something like “fellow-“ or “-mate” or “co-“ in English. So, “kaklase” = fellow classmate, “katrabaho” = fellow worker / officemate / co-worker, etc. “Kasama” means something like “fellow person joining”.
About the stress, “pakisamahin” is the infinitive form of this “-IN” verb. You probably know the infinitive form is also the one used to give commands, which is what you’re doing with the person bagging your stuff.
If you look at a bunch if “-IN” verbs, you’ll notice a pattern where typically the stress shifts one spot to the right in both the infinitive and the contemplative/future aspect (but not the other two conjugations).
That’s what’s happening here...”s
ama” turns into “pagsam
ahin”. This shift happens in most -IN verbs.
For example, try looking in the dictionary for these -IN verbs and seeing how the stress shifts relative to the root: tanungin, sagutin, kausapin, bilangin, inumin, etc.
The stress shift in those tenses/aspects start to feel natural in time.
If you want, you can practice using the -IN flashcards at the link below, which includes the most common -IN verbs:
www.tagaloglessons.c om/flashcards/in_ver bs.php