You might have seen a lot of sentences in Spanish that uses "a" as a preposition, so I am here to tell you when to use "a".

1. To indicate motion/location.

e.g. Llegamos Argentina. (We arrived in Argentina.) 

2. Before an infinitive.

e.g. Comenzó ballar. (She began to dance.) 

3. Indicate manner/mehod

e.g. Vamos pie. (We are going on foot.) 

4. Introducing an object (name).

e.g. Veré Mary. (I will see Mary.) 

Le robo una camisa Jorge. (I am taking a shirt from George.) 

5. Time Expressions

e.g. Salimos las cuatro. (We are leaving at four.) 

Got it? If not, ask me questions in the comment below! :) 

¡Que tengas un buen día!