Learn Spanish (Latin America): Lo importante es comprar una casa con garaje. - The important thing is to buy a house with a garage.

Here, lo is a neuter article used with an adjective to mean "the [adjective] thing". So, lo importante translates perfectly to "the important thing". You will hear this pattern constantly in Spanish, rather than people saying la cosa importante. Other common examples are lo bueno (the good thing) and lo mejor (the best thing).

Why do we use the infinitive comprar right after the verb es?

In Spanish, when an action is being equated to a noun or is the core focus of the sentence, it stays in the infinitive. Here, comprar translates to "to buy" or "buying" (The important thing is to buy...). You do not need to conjugate it because no specific person is actively performing the verb in this exact clause; it is just the general idea of buying.

Since this lesson is about the subjunctive, could I say una casa que tenga garaje instead of una casa con garaje?

Yes, absolutely! Because you are looking for a house that fits a certain description but might not exist or hasn't been found yet, you would use the subjunctive for the unknown antecedent: una casa que tenga garaje (a house that has a garage). The phrase una casa con garaje (a house with a garage) is just a simpler shortcut to express the exact same requirement without needing a second verb!

Why does it say con garaje instead of con un garaje?