So I was playing a game in Japanese, and I saw this sentence:
「あなたは装備し終った」
"You finished equiping" (?)
And I have seen 「し始めました」 and things like that before, and other times when they take the -masu stem of a verb and put another verb on the end, with komu etc. I was wondering, how does that work? Can you do that with any verb, or only certain compounds? How is different from using the -te form and then having the second verb?

Thanks for any help.

P.S. what is the reading of 終った in し終った? Is it しまった?

You can do that with most verbs. It is おわった.

The reading is probably しおわった and I think it means that you the results of what you finished can be seen, i.e., you finished some task.

it is する's 連用形 + 終わった. It's the same construct as 食べ終わった (Finished eating) etc.
If it was 〜て終わった it would be a sequence of events, not one verb modifying the other.

て is actually a conjunctive particle like と (that attaches to a verb's 連用形. Yes, て form is the same as 'masu' form, with some historical sound changes), so it functions somewhat like "and then".
ex: 食べて帰った = I ate and then went back (home).

If you just take the 連用形 without using て and then stick a verb onto the end of it, one ends up modifying the other to form a single action.
食べ始めた began eating
食べ終わった finished eating
食べ切れた completely ate
飛び込む jump in(to)
持ち帰る carry home
持ち歩く carry around

(note: 連用形 is what many English books on Japanese grammar books call the -masu form. It has nothing to do with masu so I don't use the term. Defining a form by telling the learner to conjugate something and then chop the end off is silly and circular.)

Yeah I learned it was called the -masu form somewhere, but now I know . Thanks everyone, that was informative.

It seems like this should be looked at as 装備し(装備する) instead of し終わった. Right? し just being another form of する。And then the 終わった part modifies the verb.

Amset Wrote:times when they take the -masu stem of a verb and put another verb on the end, with komu etc. I was wondering, how does that work? Can you do that with any verb, or only certain compounds?

One of the books on my bookshelf ('Handbook of Japanese Compound Verbs') is dedicated solely to this kind of verb combination. I think you can divide them into two classes:

(1) 'auxiliary verbs', where you can basically add a verb to the 連用形 of more or less any other verb to produce something with the obvious meaning. ~はじめる、 ~おわる、 ~きれる etc all fall into this class.

(2) 'compound verbs', where the total meaning is more than the sum of its parts. For example: 見上げる from 見る to see + 上げる to raise, meaning 'to look up', or 落ち着く from 落ちる to fall + 着く to attach, meaning 'to settle down, become calm'. There are a number of common ways the verbs can be related ('manner + action', 'action + direction', 'two simultaneous actions', etc) but no overall rules.

You'll probably find category (1) things in grammar references and category (2) things in dictionaries. I tend to think of (2) as being probably a similar degree of complexity and function to English's phrasal verbs (eg 'give up'/'give in'/'give out'...)