필요하다([pil-yo-ha-da/need]) means you must have something, it is essential. In polite speech, say 필요해요([pil-yo-hae-yo]).
필요했어요([pil-yo-hae-sseo-yo/needed]) is the polite past form. Example: 어제 필요했어요 (I needed it yesterday).
필요해야 해요([pil-yo-hae-ya hae-yo/have to need]) means it must be essential. Example: 이게 필요해야 해요 (This has to be needed).
필요하다([pil-yo-ha-da/need]) means "to need". It is used when you must have something.
1. Daily: “필요해요? (pil-yo-hae-yo)”
2. Past: 필요했어요([pil-yo-hae-sseo-yo/needed]) needed.
3. Duty: 필요해야 해요([pil-yo-hae-ya hae-yo/have to need]) have to be needed.
4. Warning: 필요하지 마세요([pil-yo-ha-ji ma-se-yo/don't need]) don't need.
A: What do you need?
B: 돈이 필요해요. B: do-ni pil-yo-hae-yo.B: I need money.
A: 돈이 필요해요? A: do-ni pil-yo-hae-yo?A: Do you need money?
B: 네, 필요해요. B: ne, pil-yo-hae-yo.B: Yes, I need it. - 필요하다([pil-yo-ha-da/need])
A: Did you need it yesterday?
B: 네, 필요했어요. B: ne, pil-yo-hae-sseo-yo.B: Yes, I needed it.
A: 얼마나 필요했어요? A: eol-ma-na pil-yo-hae-sseo-yo?A: How much did you need it?
B: 많이 필요했어요. B: ma-ni pil-yo-hae-sseo-yo.B: I needed it a lot. - 필요했어요([pil-yo-hae-sseo-yo/needed])
A: When does it have to be needed?
B: 지금 필요해야 해요. B: ji-geum pil-yo-hae-ya hae-yo.B: It has to be needed now.
A: 정말 필요해야 해요? A: jeong-mal pil-yo-hae-ya hae-yo?A: Does it really have to be needed?
B: 네, 필요해야 해요. B: ne, pil-yo-hae-ya hae-yo.B: Yes, it has to be needed. - 필요해야 해요([pil-yo-hae-ya hae-yo/have to need])
• 필요하다([pil-yo-ha-da/need]) need
- 필요했어요([pil-yo-hae-sseo-yo/needed]) needed
- 필요하고 싶어요([pil-yo-ha-go si-peo-yo/want to need]) want to need
- 필요해야 해요([pil-yo-hae-ya hae-yo/have to need]) have to need
- 필요하지 마세요([pil-yo-ha-ji ma-se-yo/don't need]) don't need