"I think I am in love!"

Translation: Jag tror att jag är kär!

[GeorgeFerguson93]

Can't I say 'jag tänka'?

[Arnauti]

1. tänka is the infinitive, so saying that will make you sound like Tarzan.
2. Jag tänker att jag är kär means roughly I am having this thought: I am in love. So it doesn't really mean the same thing.

[Lundgren8]

I explained some of the differences here!

[Andrew528568]

Shouldn't "tycker" be accepted as the English is "think" and not "believe"?