Learn French: Tu as rangé les ouvre-boîtes ? Je les ai cherchés ! - Did you put away the can openers? I looked for them!

Ouvre-boîtes is a compound noun made of a verb (ouvre) and a noun (boîtes). In French, the verb part of a compound noun never changes in the plural. The noun part takes an -s when the whole compound noun is pluralized.

Why is there an -s at the end of cherchés?

This is because of the preceding direct object rule. In the passé composé with avoir, the past participle must agree with the direct object if that object comes before the verb. Here, the pronoun les (referring to the masculine plural ouvre-boîtes) comes before ai cherchés, so we add an -s.

If that's the case, why doesn't rangé also have an -s?

Because its direct object, les ouvre-boîtes, comes after the verb as rangé. You only make the past participle agree with an avoir verb if the object appears earlier in the sentence.

Why isn't there a word for 'for' in the second sentence, like 'Je les ai cherchés pour'?

The French verb chercher inherently means 'to look for' or 'to seek'. You do not need to add an extra preposition. Translating 'for' literally by adding pour is a very common trap for English speakers!