Breakdown of Мне понадобилась твоя помощь.
я
I
твой
your [informal]
помощь
the help
понадобиться
to be needed, to come in handy (perfective)
Questions & Answers about Мне понадобилась твоя помощь.
Why is the verb понадобилась in the feminine form?
In this structure, the grammatical subject is actually помощь (help), which is a feminine noun. Just like with the verb нравиться (to like), the sentence is literally structured as 'Your help became necessary to me.' Therefore, the verb agrees with the thing needed, not the person needing it.
Why do we use the Dative Мне instead of the Nominative Я here?
This sentence uses a 'Dative experiencer' structure, much like the impersonal predicates пора (it's time) or лень (too lazy) from this lesson. With verbs of need like понадобиться, the person experiencing the need is put in the Dative case to show to whom the thing became necessary.
How does понадобилась differ from just saying мне нужна была?
While мне нужна была describes a continuous state in the past ('I was in need of'), the perfective verb понадобилась focuses on the onset of that state or a specific event ('I suddenly needed' or 'the need arose').
The word order here is Verb-Subject (понадобилась твоя помощь). Why is it arranged this way?
Putting the verb before the subject is very common in Russian when introducing an event that occurred. You could say Твоя помощь понадобилась мне, but that would put a strong, contrastive emphasis on 'to me' (as opposed to someone else). The current word order is the most natural way to state the fact that the need arose.