Learn Spanish (Latin America): El carro de mi cuñado es tan rápido como el de mi cuñada. - My brother-in-law's car is as fast as my sister-in-law's.

In English, we just say "my sister-in-law's" and leave out the word "car". In Spanish, you can't end a sentence with a possessive word like that. Instead, you use a definite article (el, la, los, or las) to replace the noun you want to avoid repeating, followed by de. So el de mi cuñada literally means "the one of my sister-in-law".

How do I know whether to use el de, la de, los de, or las de?

The article has to match the gender and number of the noun you are replacing. Here, el replaces el carro (masculine, singular). If we were comparing motorcycles (las motos), the sentence would end with las de mi cuñada (literally, "those of my sister-in-law").

I always mix up tan and tanto. Why is it tan rápido como here?

You use tan before an adjective (like rápido) or an adverb to mean "as [adjective] as". You only use tanto (or its variations tanta, tantos, tantas) before nouns to talk about quantities, like tanto dinero como ("as much money as"). Since rápido is an adjective describing the car, we need tan.