Learn Spanish (Spain): ¿Vas a comprar el vestido? Sí, voy a comprarlo. - Are you going to buy the dress? Yes, I am going to buy it.

In Spanish, lo is a direct object pronoun meaning 'it', and here it replaces the masculine noun el vestido (the dress). Because comprar is an infinitive (an unconjugated verb ending in -ar), you are allowed to attach the pronoun directly to the end of it to make one single word.

Could I put the 'lo' somewhere else in the sentence?

Yes, you can! When you have a conjugated verb followed by an infinitive, like voy a comprar, you have the choice to place the pronoun before the conjugated verb. Saying Sí, lo voy a comprar is also perfectly correct and means exactly the same thing.

How does the phrase 'vas a comprar' work to mean 'going to buy'?

To talk about plans in the near future, Spanish uses the present tense of the verb ir (to go) plus the word a plus an infinitive verb. Vas is the 'you' form of ir, so vas a comprar literally means 'you go to buy', which is how Spanish expresses 'you are going to buy'.

Why aren't the words for 'you' or 'I' used in this sentence?

Spanish often drops subject pronouns like tú (you) and yo (I) because the verb forms themselves tell you who is acting. The verb vas can only ever mean 'you go' and voy can only ever mean 'I go', so adding the pronouns here is not necessary.