Even for me(native japanese), it's difficult to explain the grammar of すみません. You can just remember it as a word I think. すみません means "Excuse me". When you want to say "Excuse my poor Japanese" (? Is this what you wanted to say??), you can say like 下手な日本語ですみません. 下手な is "poor(adj)". で is preposition. This preposition needs to be changed to other ones depends on the sentense. For example 私の犬がすみません。"Excuse about my dog". You need to use が as a preposition . Another example , 私の犬が(あなたの)邪魔をしてすみません。"Excuse about my dog bothers you. 邪魔をする means "bother(verb)". You need to change the end of word from する to して for connecting the sentences "邪魔をする" and "すみません". Btw すみません is formal form, and すまん is casual form. But すまん sounds like man and even sometimes rude. So I recommend to use ごめん(casual form of I am sorry) instead of すまん or すみません to your friends. I hope this helps:-)

Yes. すみません is a verb. This is (私の気が）すみません。 済む（すむ）　→　済まない 済む：finish, stop, nothing left unfinished. 気がすむ：feel OK in mind. すみません：I don't feel OK in my mind. -> I don't feel OK in my mind (I'm asking this ,doing this) -> Excuse me. ※ Origin of 済む、住む、澄む （all SUMU) is 澄む（water gets clear and transparent). あなたは私の日本語がすみません -> should be said 日本語が下手ですみません。 　　　　－＞ I don't feel OK in my mnd because my Japapese is poor －＞ Sorry for my poor Japanese.

ah, im not in the place to correct anything, but i've seen someone write something along the lines of: 私の日本語は下手くそだ。**"my japanese is bad." i guess you can just add a "ごめんなさい" at the beginning or the end¿ i apologize because i really am a beginner in japanese, but i hope this answered at least one of your questions. ** p.s: i think the sentence is informal.