Mandarin/Eng "I think, thought"

1) I think.
我想。

1.

think= 想
I think of you.(我想起你)

I think a way to solve the problem.(我想到解决问题的方法了)

2.

think=认为

a:Can you guess my carrer?

b:I think you are a teacher.(我认为（觉得）你是一个老师。)

2）I think you are a American person. (I think you are American.)
我想你是美国人。

3) You think she is American?
你想她是美国人吗？

This is a correct way to say like that , but not a very chinese way.

Here "认为" is better than "想"
4) You have 2 older sisters?
你有两个姐姐（吗）？

“吗” is a modal particles, usually in the end of the qurstion sentence.

5) I think you have 2 younger sisters.
我觉得你有两个妹妹。

6) I thought.
我想了。

“Thought” means you thought about it before you say it.

"我原以为" sounds better.

For example:

1.think=想了

a: Did you really think about this question

b:yes,i thought (是，我想了)

PS：“想了” means you think about a thing,especially a question or a view of something.

 Mostly you need make a decision or an answer after you "think(想了)" 

2.think=以为，thought=原以为

a:really ? you are a chinese,I thought you are a Korean.

(真的吗？你是一个中国人，我原以为你是韩国人)

PS：“以为” means it's your own idea of something, it's just a way to show your view after you know the right answer.

7) I thought you were American.
我想了你是美国人。

(我原以为你是美国人)，“原以为” in here means you already know "you" are not an american,but you want "you" to know your orginal opinion.

You know how to use the right word already but I want  it will help you to understand them well.

Fighting. :)

Mandarin/Eng "I think, thought"

1) I think. ——我觉得or我认为（猜测性质，不太敢肯定）
我想。 


2）I think you are a American person. (I think you are American.)
我想你是美国人。

3) You think she is American?
你想她是美国人吗？

4) You have 2 older sisters?
你有两个姐姐？

5) I think you have 2 younger sisters.
我觉得你有两个妹妹。

6) I thought. ——我以为（对所猜测的事情持有肯定性，把握）
我想了。  

7) I thought you were American.
我想了你是美国人。

Mandarin/Eng "I think, thought"

1) I think.
我想。 (or我认为)


2）I think you are a American person. (I think you are American.)
我想你是美国人。（认为，觉得） 

3) You think she is American?
你想她是美国人吗？ (同上)

4) You have 2 older sisters?
你有两个姐姐？

5) I think you have 2 younger sisters.
我觉得你有两个妹妹。

6) I thought.
我想（思考过）了。

7) I thought you were American.
我想了你是美国人。（还以为）