Learn French: Le congrès auquel tu participes est très intéressant. - The convention you are participating in is very interesting.

The verb in this sentence is participer à (to participate in). Because it requires the preposition à, we can't just use que. We have to combine à with the relative pronoun lequel (which). Whenever à and lequel meet, they must contract into the single word auquel (literally 'to which').

Why does auquel translate to 'in which' here instead of 'to which'?

Prepositions rarely translate word-for-word between languages. In English, you participate 'in' an event, but in French, the verb is always participer à. Since the French verb dictates the preposition, we must use à, which becomes auquel here, even though the English translation uses 'in'.

How do we know to use auquel and not à laquelle or auxquels?

The relative pronoun must agree in gender and number with the noun it refers back to. Here, it is replacing Le congrès (the convention), which is masculine and singular. If it referred to a feminine noun like la conférence (the conference), you would use à laquelle.

The English translation puts 'in' at the very end. Can we do something like that in French?