HSK 3 grammar · pattern
是（强调）
是 — emphatic 是 ("I really do / it really is")
Pattern
+ +
是 is stressed
+ + +
( ) can be left out · other words may come before, between and after
How it works
Putting 是 in front of a verb or adjective insists that the statement really is so. It answers a doubt rather than adding meaning, and it is spoken with the stress on 是: 我是不知道 "I really don't know". Drop it and the sentence still works, which is the quickest way to tell it apart from the 是⋯⋯的 cleft — that one needs its 的 and points at when, where or how something happened.
Examples
我是不知道这件事，你别问我了。
Wǒ shì bù zhīdào zhè jiàn shì, nǐ bié wèn wǒ le.
I really don't know about this — stop asking me.
今天是冷，你多穿一件吧。
Jīntiān shì lěng, nǐ duō chuān yí jiàn ba.
It really is cold today; put on another layer.
他做的菜是好吃，就是上得太慢了。
Tā zuò de cài shì hǎochī, jiùshì shàng de tài màn le.
His food really is good — it just comes out too slowly.
Related
See it in a story
In the app, 是（强调） is underlined wherever it appears in the stories you read, and practised with sentences from them. Free.