Learn Portuguese (Portugal): Eu preciso da tua ajuda.

The verb precisar (to need) is followed by the preposition de when you need a noun. Here, the noun phrase is a tua ajuda (your help). When the preposition de meets the feminine article a, they combine to make da (de + a = da).

Why do we need the article a before tua? Doesn't tua already mean your?

Yes, tua means your, but in European Portuguese, you almost always put a definite article (o or a, meaning the) before a possessive word. Because ajuda is feminine, we use a tua. That is why the de from precisar merges with it to become da tua.

Does precisar always need the preposition de?

It needs de when you need a thing or a person (a noun). For example, eu preciso de tempo (I need time). But if you need to do an action (a verb), you generally do not use de. For example, eu preciso dormir (I need to sleep).

Can I drop the word Eu and just say Preciso da tua ajuda?

Yes, absolutely! Because the verb ends in -o (preciso), it is already clear that I am the one who needs help. Dropping the pronoun is very common in Portuguese.