What tense?

present

The statement "cats climb trees" is related to an unasked question in the present tense "what do cats do ?". The fact that this stated property of cats is good for future predictions of their behaviour is perhaps what is confusing you.

Thanks for taking time to answer my question. It was very helpful. In a sentence like "Cats ___ (to climb) trees." , I was wondering if the "only" answer that would be grammatically correct would be "climb". If I used "climbed", would there then be a need to use articles? Example: The cats climbed the trees. In other words, does the sentence require more information to elicit the correct answer? Example: "Cats ___ (to climb, present) trees." Thanks again for the helpful feedback.

No. You would say:

Cats have been climbing trees for years.
Cats have climbed trees for centuries.
Cats still climb trees. They climb trees all the time.
Cats will always climb trees.
They also scratch things and have been scratching things for years.

You do not need an article.

Obviously there is a difference in focus when an article is introduced. No article ->> the general situation. Mame's examples illustrate that general condition continues. When an article is introduced, the focus shifts to a specific situation.

If you want to be specific:

My cats climbed these trees last year and this rose bush this year.

My cat climbs the tree I planted whenever I'm not home.


Hope that helps.