생각하다([saeng-gak-ha-da/think]) means an idea is moving in your head. In polite speech, say 생각해요([saeng-gak-hae-yo]).
생각했어요([saeng-gak-hae-sseo-yo/thought]) is the polite past form. Example: 그렇게 생각했어요 (I thought like that).
생각해야 해요([saeng-gak-hae-ya hae-yo/have to think]) means your brain must work. Example: 다시 생각해야 해요 (I have to think again).
생각하다([saeng-gak-ha-da/think]) means "to think". It is used when an idea moves in your head.
1. Daily: “뭐 생각해요? (mwo saeng-gak-hae-yo)”
2. Past: 생각했어요([saeng-gak-hae-sseo-yo/thought]) thought.
3. Duty: 생각해야 해요([saeng-gak-hae-ya hae-yo/have to think]) have to think.
4. Warning: 생각하지 마세요([saeng-gak-ha-ji ma-se-yo/don't think]) don't think.
A: What are you thinking?
B: 일 생각해요. B: il saeng-gak-hae-yo.B: I'm thinking about work.
A: 어떻게 생각해요? A: eo-tteo-ke saeng-gak-hae-yo?A: What do you think?
B: 좋아요. B: jo-a-yo.B: I think it's good. - 생각하다([saeng-gak-ha-da/think])
A: What did you think?
B: 그렇게 생각했어요. B: geu-reo-ke saeng-gak-hae-sseo-yo.B: I thought like that.
A: 언제 생각했어요? A: eon-je saeng-gak-hae-sseo-yo?A: When did you think?
B: 어제 생각했어요. B: eo-je saeng-gak-hae-sseo-yo.B: I thought yesterday. - 생각했어요([saeng-gak-hae-sseo-yo/thought])
A: What do you have to think about?
B: 다시 생각해야 해요. B: da-si saeng-gak-hae-ya hae-yo.B: I have to think again.
A: 왜요? A: wae-yo?A: Why?
B: 결정해야 해요. B: gyeol-jeong-hae-ya hae-yo.B: I have to decide. - 생각해야 해요([saeng-gak-hae-ya hae-yo/have to think])
• 생각하다([saeng-gak-ha-da/think]) think
- 생각했어요([saeng-gak-hae-sseo-yo/thought]) thought
- 생각하고 싶어요([saeng-gak-ha-go si-peo-yo/want to think]) want to think
- 생각해야 해요([saeng-gak-hae-ya hae-yo/have to think]) have to think
- 생각하지 마세요([saeng-gak-ha-ji ma-se-yo/don't think]) don't think