Learn Spanish (Spain): ¿Se conocen desde la infancia? - Have they known each other since childhood?

Desde is used with a specific point in time or an era, like la infancia (childhood), meaning "since". You would use desde hace if you were talking about a duration or amount of time, like desde hace diez años (for ten years).

The English translation says "Have they known", so why does the Spanish sentence use the present tense conocen?

In Spanish, when an action started in the past and is still happening right now, you use the present tense with words like desde. Because they still know each other today, we use the present tense conocen, whereas English uses the present perfect "have known".

What is the word se doing in this sentence?

Here, se is a reciprocal pronoun meaning "each other". Without it, conocen just means "they know". Adding se makes it se conocen, meaning "they know each other".

Why is there a la before infancia? We don't say "since the childhood" in English.

Spanish often requires a definite article (el, la, los, las) when talking about general concepts, stages of life, or abstract ideas. Because of this rule, "childhood" becomes la infancia.