Forming passé composé sentences

Passé composé is a very useful tense in any language. It describes what has happened in the past. 


To form these sentences you either use the verb 'avoir' or the verb 'être'
'Avoir' means 'to have' and  'être' means 'to be'


Once you have put the verb, you add in something to the ending, also known as the 'past participle' 
In most cases, the 'past particple' for -er verbs is an 'é'
So, you replace the 'er' at the end of the verb, and then to replace it you add an 'é' 


For example: 
I bought. 
the verb 'to buy' is 'acheter' 
In present tense : I buy =  J'achète
So in order to make it passé composé, you replace the -er and add an é. And remember to either have an  'avoir' or 'être' verb.
In passé composé : I bought = J'ai acheté


There are also irregular verbs. There is not particular pattern with these verbs, you just have to memorize them. 


An example is: 
I drank. 
Present tense: Je bois
Passé composé: j'ai bu