To tell the -er infinitive from the -é past participle, replace the first-group verb with a third-group verb like "vendre/vendu". If "vendre" sounds right, it is the infinitive (-er). If "vendu" sounds right, it is the participle (-é).
Replace with "vendre/vendu". Vendre: -er. Vendu: -é.
Examples
Je vais manger.
Key word(s): manger
"Je vais vendre" works, so it is the infinitive -er.
J'ai mangé.
Key word(s): mangé
"J'ai vendu" works, so it is the participle -é.
Il faut travailler.
Key word(s): travailler
"Il faut vendre" works, so it is the infinitive -er.
La porte est fermée.
Key word(s): fermée
"La porte est vendue" works, so it is the participle -é(e).
Sans hésiter, il a parlé.
Key word(s): hésiter, parlé
"Sans vendre" (infinitive) plus "il a vendu" (participle).
Common mistakes
Writing "j'ai mangé" with -er
Writing "je vais manger" with -é
Hesitating after a preposition (pour, sans, à, de always take the infinitive)
Test yourself
Il est allé ___.
"Il est allé vendre", so the infinitive is used.
Le gâteau est ___.
"Le gâteau est vendu", so the participle is used.
Elle a ___ de ___.
"A vendu" (participle) plus "de vendre" (infinitive).
Tips to remember
After a preposition (à, de, pour, sans, par), always use the infinitive (-er).
After "avoir" or "être", always use the past participle (-é).
The "vendre/vendu" trick works every time.
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