what does ga mean in this sentence??
You didn’t say which sentence…
Generally, が is a subject marker. (However, it’s better to think of が as an ‘identifier marker’, since we often think of ‘subjects’ in English differently from grammar meaning. ‘Identifier marker’ indicates the speaker is identifying, or wanting to identify, something unspecified eg. Who [ga] is she?).
が is also a conjunction meaning “but” (It joins two sentences.)
So I’m guessing you were wondering how が is used in: もうしわけ あり ませ ん *が * おしえる こと は でき ませ ん 。= I am sorry BUT I cannot tell (you who she is).
[おしえる = teach, show, tell]
あなたに似ている人がいます。
あなたに似ている … resembles you
あなたに似ている人 a person who resembles you
あなたに似ている人がいます。 A person who resembles you exists. → There is a person who resembles you.
“象は鼻が長い” means that elephants have a long trunk.
If you want to understand the sentence in Japanese word by word, you can paraphrase it in English as follows:
象は、(Speaking of elephants, )
鼻が長い。( their trunk is long.)
Speaking of, or as to elephants, their trunk is long.