Learn Farsi: این هدیه واسه پسره‌ست.

واسه is the conversational, everyday version of برای (for). In spoken Farsi, you will hear واسه much more often than برای, though they mean exactly the same thing.

How does پسر (boy) become "the boy" in this sentence?

In formal Farsi, there is no direct word for "the". However, in spoken Farsi, you can add an -e sound (written as the letter ه) to the end of a noun to make it specific. So, پسر means "a boy", but پسره means "the specific boy we are talking about".

What does the ست (-st) at the end of پسره‌ست mean?

It is a contraction of است (ast), meaning "is". Because پسره ends in a vowel sound, the word است drops its first letter and attaches directly to the noun, making the word پسره‌ست (pesare-st).