There are many words in Klingon that are used to translate the English word "think":

HIt - This verb is used to describe talking to yourself in your mind. It is used like a verb of speech.

Qapla' vIchavta'. HIt HoD.
"I have achieved access", the captain thought to herself. 

jal - This verb is used to describe picturing things in your mind.

targh SuD yIjalQo'!
Don't think about a green tagh!

qel - This verb is used to describe thinking about a topic.

Hoch Qapla' wIqel.
We thought about every success. 

paQ - This verb is used to describe thinking deeply about a topic or in general.

batlh paQ
She thought deeply about honor. 

-law' - This suffix is used to express uncertainty on the part of the speaker. This is "think" as in "suspect".

nughochlaw' Dujvetlh.
I think that ship is tracking us.

Qapla' Dachavlaw'ta'.
I suspect that you have achieved success.

Har - This verb is used to describe holding an opinion that you have accepted as true.

Qapla' vIchavta' 'e' DaHar'a'?
Do you think that I have achieved success?

Hon - This verb is used to describe disbelief about a topic. This is included as it's the opposite of Har. 

Qapla' Dachavta' 'e' vIHon.
I doubt that you have achieved success.

SIv - This verb is used to describe curiosity about a topic. This is included as it's a type of "thinking" activity. 

Qapla' Dachavta' 'e' vISIv.
I am curious about whether you have achieved success.

Qub - This verb is used to describe what the mind does in general. 

buSHach vIQub
I think thoughts