When think means ‘have an opinion’, it cannot be used in the progressive form.

‘What do you think of my hair color?’ ‘Frankly, my dear, it is a disaster.’ (NOT What are you thinking of my hair color?)

I don’t think that we will win the election. (NOT I am not thinking that we will win the election.)

When think means ‘plan’ or ‘consider’, progressive forms are used.

What are you thinking about? (NOT What do you think about?)

After think, we use an –ing form, not an infinitive.

I am thinking of writing a novel. (NOT I am thinking to write a novel.)

In a very formal style, think can be followed by an object + (to be) + complement.

I thought her interesting.

We thought it intriguing.

We thought him an idiot.

To be is sometimes used before the complement.

We thought him to be a fool.

In a less formal style, we use that-clauses after think.

I thought that she was interesting.

We thought that it was intriguing.

We thought that he was an idiot.

We do not usually use think to introduce indirect questions.

I was wondering if you could lend me a pound. (NOT I was thinking if you could lend me a pound.)