used to talk about something already known or mentioned
The cat I saw yesterday was black.
昨日私が見た猫は黒かった。
used for the only or main one of its kind
The sun is very bright today.
今日の太陽はとても明るい。
used to specify which person or thing you mean
The book on the table is mine.
テーブルの上のその本は私のものです。
used to talk about something in general, not a specific one
The piano is a beautiful instrument.
ピアノは美しい楽器です。
used with adjectives to refer to a group of people or things
We should help the poor.
私たちは貧しい人々を助けるべきです。
used before a family name to refer to the whole family
The Smiths are coming to dinner.
スミス家が夕食に来ます。
used with units to mean 'every'
She gets paid by the hour.
彼女は時間給で支払われます。
enough of something for a purpose
I don't have the time to help you.
あなたを助ける時間がありません。
used with time to mean 'the present'
This is the song of the day.
これは今日の歌です。
used to show that something is famous or important
She met the president of the company.
彼女は会社の社長に会いました。