필요하다, 필요 있다, 필요로 하다 : what's the difference in meaning and usage?

Question

I was aware of '필요하다' '필요 있다' both meaning 'to be necessary', and on Naver Dictionary saw that the first translation of 'need' is '필요로 하다'.

What is the difference in meaning and usage, if any, between these?

I am confused about the naver entry as I thought 필요하다 was intransitive, meaning 'to be necessary' rather than 'need', so 내가 X을 필요해 would be wrong. However, the entry is given as

(…을) 필요로 하다

Does the '(…을) imply that '필요로 하다' is transitive? It seems so from some of the naver examples, e.g.

Intransitive, I think:

도움이 필요하세요?

가지 마. 네가 필요할지도 몰라.

But this seems transitive:

그들은 변화를 대단히 필요로 하고 있었다.

I think I may have heard 필요해 하다 as a transitive form. Is that the same as 필요로 하다?

Answer 1

In the example "(을) 필요로 하다, the word with "-을" will be the object of 하다; however, this doesn't imply that the equivalent phrase 필요하다 will also take an object. Rather, the object of -을 필요로 하다 is the subject of 필요하다.

According to the dictionary, 필요하다 is not an intransitive verb, but an adjective (or adjectival verb or descriptive verb or whatever you want to call it; 형용사 in the dictionary). So I guess you could rewrite your last example as:

그들은 변화가 대단히 필요해요.

I don't think there's much difference in meaning between these.

As for 필요가 있다, it is normally used with a verb: it is necessary to ~ / there is a need to ~:

그 사람들을 도울 필요가 있다 (It is necessary to help these people / [we] need to help these people).

Answer 2

필요하다

is the basic transitive verb. For example:

나는 저게 필요해 (from 나는 저것이 필요하다). (I need that.)

It may however look like, or indeed be, an intransitive verb in:

우리에게는 웃음이 필요하다. (To us laughter is needed.)

--which is (almost) identical in meaning with:

우리는 웃음이 필요하다. (We need laughter.)

These may reveal the limitation of trying to apply a concept like transitivity to Korean verbs (or trying to say which of 우리는 웃음이 should be the subject of the sentence). See this other answer.

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필요로 하다

is used mostly for relationship of necessity between things, as in:

축구나 야구 같은 운동은 넓은 운동장을 필요로 한다. (Sports such as soccer and baseball require a large playing field.)

That doesn't mean anybody wants to play soccer or baseball or needs a field. It is just a fact about what ball playing entails.

A similar construction would be:

6시 퇴근을 원칙으로 한다. (Leaving work at 6 shall be the principle.)

Basically, 으로 하다 confers a status.

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필요해 하다

is a reference to someone's felt or exhibited need as in:

그는 가족의 사랑을 많이 필요해 한다. (He is--feels or acts, however without any suggestion of insincerity--in great need of the love of family.)

Notice that '사랑이 필요하다' has turned into '사랑을 필요해 하다.'

Another example of the same construction would be

그는 요즘 퇴근 후에 많이 피곤해 해. (Nowadays after work he is--etc.--very tired.)