Learn Russian: Этот телефон старый, зато у него есть одно преимущество. - This phone is old, but on the other hand it has one advantage.

While но just means "but" and states a simple contrast, зато specifically shows compensation. It translates closely to "but to make up for it" or "but on the plus side." The phone being old is a negative, but the advantage balances it out.

Why does the word for "it" (он) become него instead of его after the preposition у?

In Russian, whenever a third-person pronoun (like он, она, оно, они) comes directly after a preposition, an extra н- is added to the beginning. This is a strict rule that just makes the transition between the words easier to pronounce!

Why is the number one spelled as одно in this sentence?

The word for "one" has to match the gender of the noun it is counting. преимущество (advantage) ends in -о, making it a neuter noun. Therefore, we must use the neuter form одно instead of the masculine один or feminine одна.

Just to check my pronunciation, how should I say the г in него?