usar las tarjetas, prevenir contagios

Question

Hola Inma,".... usar las tarjetas, ..... prevenir contagios"can you, please, explain why do you use the article in the first case but not in the second? I thought articles in Spanish are used when talking in general, which apply in both cases (I think).Muchas graciasΕλισάβετ

Answer

Hola Ελισάβετ
Both these sentences could either include or omit the articles. As a very general rule, the use of the definite article is seen when we want to be a bit more specific about the noun, and we tend to omit it when we talk about a noun in a very general way. For example, in the second sentence we haven't used the article because we are referring to "contagion" in general; this is why we used "para prevenir contagios". 
As for the first sentence "usar las tarjetas de crédito", you could easily drop the article and the meaning would be the same: "usar tarjetas de crédito". This is a choice for the narrator, really. At the time of speaking the narrator can express this with or without the article depending on how general they want to be. 
The reason why we don't have a specific lesson on this and we are not testing on this topic is because this is a very general rule and it goes very often with what nuance the speaker wants to express. 
It is one of the most difficult grammar points as we don't have a clear/easy to follow rule here. It's often subjective.
Inma