The verb think is not normally used with an infinitive.

You can say, ‘I want to do something’, but you can’t say ‘I think to do something.’

The verb think has two meanings.

When think means ‘consider’, it is almost always followed by an –ing form.

Two structures are possible.

• think of doing something
• think about doing something

I am thinking of buying Sam’s car. (NOT I am thinking to buy Sam’s car.)

They are thinking about adopting a baby. (NOT They are thinking to adopt a baby.)

When think means ‘to believe’, it is usually followed by a that-clause.

A lot of people think that their sex appeal will increase if they lose weight.

That can be dropped in an informal style.

She thinks she is the best candidate for the job. OR She thinks that she is the best candidate for the job. (NOT She thinks to be the best candidate for the job.)

However, the infinitive can be used in the passive structure.

Thousands of children are thought to have died in the battle. (= Thousands of children are believed to have died in the battle.)

Note that when think means ‘believe’, it is not normally used in its –ing form.

I think we can trust him. (NOT I am thinking we can trust him.)

She thinks she has found a solution to the problem. (NOT She is thinking she has found …)