How do I form the past simple tense in Spanish?

Spanish verbs (words like 'to do' or 'to say' or 'to eat', in English) come in two main groups. Firstly, there are the -ar ending verbs, which make up about 70 per cent of Spanish verbs. Some examples of these are 'comprar' (to buy), 'enviar' (to send), and 'hablar' (to speak). Secondly, there are the -ir and -er ending verbs, such as 'vender' (to sell), 'compartir' (to share) and 'entender' (to understand).

Forming the past simple is easy for regular Spanish verbs, just do the following:

For -ar ending verbs:

I bought: compré

He/She/It bought: compró

They bought: compraron

You bought: compraste 

N.B. 'We bought' is exactly the same in the past tense as it is in the present: 'compramos'

Easy, right?

Now, for the -er/-ir verbs, simply do the following:

I understood: Entendí

He/She/It understood: Entendió

They understood: Entendieron

You understood: Entendiste

We understood: Entendimos

N.B. Remember, the -ir/-er endings are exactly the same!