I've seen some sentences end with particles like が and で. The most common particle that I see is に. For example, if someone asked:
どこにいる？ → Where are you?
I could reply,
友達{ともだち}の家{いえ}に → (I'm at) my friend's house.
I'm still lost on the particles で, and が. Help would really be appreciated if someone could explain how those two particles are used at the end of a sentence.
This is essentially an omission of a word, in this case being the verb following the particle. This is common in speech as answers to questions, and often involves omission of the main verb of a sentence.
どこにいる？
友達の家に（いる。）
誰が勝ったか？
俺が（勝った。）
何語で話しかけられた？
英語で（話しかけられた。）
It is also common, in speech, to add a similar clause after the verb has been spoken. This can be done for clarification. This pattern does not require the sentence to be an answer to a question and can just be added by the speaker in anticipation of the listener not getting the full picture.
A: 登録しました。
B: （何を？）
A: 名前を
B: （どこに？）
A: データベースに
B: （何で？）
A: SQLクエリーで
