I definitely heard that she cursed me at the cafeteria.

Better say: I definitely heard that she cursed me in the cafeteria.

He is always jealous when his girlfriend hang out with her friends.

Better say: He is always jealous when his girlfriend hangs out with her friends.

She got a bonus as a reward about long servies.

Better say: She received a bonus as a reward of her long service to the company. 

He forgot to buy souvenir for his family.

Better say: He forgot to buy souvenirs for his family. (*Use plural form of the noun "souvenir".) 

Is this the right gate of outbound flight to Korea?

Better say: Is this the correct outbound flight gate for Korea?

I heard there are lots of pickpockets in Europe.

**Good and correct sentence!~ ^^ 

They dreamed of robbing a bank and to be a rich.

Better say: They dreamed of robbing a bank to be a rich.

Did you really invest this company two grand?

Better say: Did you really invest two grand in this company? 

I paid for bill every time so I have a lot of changes.

Better say: I paid my bills on time every time, therefore I have a lot of changes.

Check your change whenever you pay.

****Good and correct sentence!~ ^^ 

I forgot cashing my traveler's checks.

Better say: I had forgotten to cash my traveler's checks.

Using envelope when you give money to someone is manner.

Better say: It is polite to give someone money in an envelope.