手早い答え
já は変化の完了、ainda は以前の状態の継続を示すことが多いです。
確実な見分け方は、英語から訳す前に、その単語の働きを見きわめることです。
それぞれの単語の働き
já
already, now, or right away
Use já for something completed earlier than expected or true by now.
ainda
still or yet
Use ainda for something continuing or not completed up to now.
選び方
- já: Choose já if the sentence points to something completed earlier than expected or true by now.
- ainda: Choose ainda if the sentence points to something continuing or not completed up to now.
例文
Eu já terminei.
I have already finished.
Eu ainda estou estudando.
I am still studying.
よくある落とし穴
In questions, já asks whether it has happened already, while ainda asks whether it remains true.
文脈の中で練習する
自分の実生活から文を一つ選び、二回書いてみましょう。一度はjáで、もう一度はaindaで。どちらも成り立ちそうなら、意味の違いを言葉にしてみましょう。