How do you know whether to use « er » or « é/ée/és/ées » at the end of a verb ?

This is a common mistake even for native French speakers ! The difference is that « er » is the infinitive form of some verbs whereas verbs ending with -é (for masculine, or -ée for feminine, -és for masculine plural and -ées féminine plural) are the past-participate forms. When deciding on which ending you should choose, you need to figure out whether you need the infinitive form or the past participate. Here are the rules:
Use the infinitive (-er):After a verb, even if it is not conjugated (EXCEPTED WHEN IT IS ETRE OR AVOIR).Je n’aime pas aller à la salle de sport. (I don’t like going to the gym)Tu devrais manger plus sainement. (You should eat healthier)After a preposition: à, pour, de, etcIl a besoin d’aide pour se lever. (He needs help to stand up.)Tu l’as aidé à préparer son entretien ? (Did you help him to prepare for his interview?)When using the infinitive form: you would use it to give advice, and instructions.Incorporer la farine, et mélanger. (Add the flour, and stir.) Ne pas toucher. (Do not touch)
Use the past-participate:When you use it as an adjectiveElle est fatiguée. (She is tired).Le temps est ensoleillé aujourd’hui. (The weather is sunny today).After the auxiliary verbs (être and avoir) Il a mangé toutes les frites (He ate all the chips).Elles ont vu la tour Eiffel. (They saw the Eiffel Tower)Elle est tombée dans l’eau. (She fell in the water)
One last rule (yes, French is a tricky language…): do not forget to « accorder le verbe » (verb agreement with its subject):With « être », the past participate needs to end with -ée with a masculine subject, -ée if the subject is feminine, -ées if only feminine plural and -és if masculine plural or both masculine/féminine plural. With avoir, verbs don’t agree with their subject. However, they require agreement with any preceding direct object (simply talking, when what you are talking about is located before the verb, the verb needs to agree with the object)Ils ont mangé de la viande crue. They ate raw meat.La viande qu’ils ont mangée était crue. The meat they ate was raw. La viande is feminine, so you need -ée. To locate the direct object, you need to ask yourself "what" or "who". Here, what did they eat ? Meat. Meat is the direct object.