Maybe you already knew that 是 is a pretty neat word. It covers all the 'am, is, are, was, were' in English. Then maybe you want to use it whenever you see 'to be' in any form.
Sometime, it's pretty safe to use it. Such as:
He is a magician. --> 他 是 魔术师。
My teacher is an American. --> 我的 老师 是 美国人。
Notice that in above sentences the words follow 'to be ' are all noun.
Let's see more example,
Tony is young. --> 托尼 是 年轻。 Oops! That's not correct. A native speaker will tell you the correct way is : 托尼 很 年轻。There is no 是 in the sentence at all. Why? When the word follows 'to be' is adjective, you don't need to translate the 'to be' into 是 at all. The Chinese way to say the sentences like " Subject + be+ adjective" is " Subject + 很 + adjective". More practice to make the rule clear? Let's move on...
You are pretty. --> You 很 pretty. --> 你 很 漂亮。
I am fine. --> I 很 fine. --> 我 很 好。
I am tired. --> I 很 tired. --> 我 很 累。
Here is one more thing. 很 literally means 'very'. But in above sentences, 很 doesn't necessarily mean 'very'. The context or the way how the person says it will tell you whether 很 means 'very' or not.
Now I am going to wrap up the rule about when you need to say 是.
If the English sentence structure is "Subject + be + noun" , you can safely do a word by word translation. Then we get the Chinese sentence structure is " Subject + 是 + noun".
If the English sentence structure is " Subject + be + adjective", the Chinese sentence structure will become " Subject + 很 + adjective"。
Now it's you turn. Please translate the following sentences into Chinese and type your answer as comments if you like.
Mike is handsome.
Mark is a pilot.
I am an engineer.
We are smart.
I am thirsty.