Modified by degree adverbs, common adjectives can serve as predicates directly without "是" , which is different from English.

EP:  她很漂亮。 （right )     她是很漂亮。（wrong)

      She is pretty.

      我困了。   (right)    我是困了。( wrong)

     I am sleepy.

You  want to know when we can use a 是 before a adj word?

<是 + adj + 的>  Non-predicate adjectives can be put into the structure slot 是...的.

EP:   我的裙子是红色的。   My skirt is red.

      是长的  /   是弯的  /  是亮的  / 是金的  /  是快的  /  是慢的  /

     you see ..all of them should have "的" .

First notebook entry: Introducing myself

我累了，and 我是累了，
which is right？

两句话在语法上来讲都是对的。Both them are right !

I am tired.  我累了

yes, I am tired.  是的，我是累了！  所以，后者应该用在“你累了吧？”这类疑问句的后面做回答。

First notebook entry: Introducing myself

我累了，and 我是累了，
which is right？

我累了is right.

First notebook entry: Introducing myself

我累了，and 我是累了，
which is right？

我累了is Common to express, exp I'm tired because of working all the day.

是的，我累了 equal 我是累了,