…是…，但是…【shì … dànshì】

A word, then 是, then the same word again, grants a point: "it's X, sure". The next clause, with 但是, 可是, 就是 or 不过, says what the catch is. The weight of the sentence falls on the catch.

The repeated word is usually an adjective. A verb works the same way: "I'll go, but…".

就是 as the "but" is soft – "it's just that…". It suits a small complaint.

Where it goes wrong

The word has to be said twice, once on each side of 是.

• × 好是，但是太贵了 → 好是好，但是太贵了

This 是 sits between two copies of the same adjective. A plain adjective sentence still takes 很, not 是.

• × 这个房子是大 → 这个房子很大