Care and maintenance of deck in the garden

I recently bought a house. There is a deck in the garden. It is about two or three years old. It does not seem to be in a great condition, but I have no experience with decks and find it hard to tell how bad it is and what I need to do in terms of care and maintenance.

Here is a photo of the deck:

You can see that there is a slightly darker shaded area right of the middle. When it rains, the water occasionally stands on it. Question 1: Is there anything I can do about that in terms of prevention or care?

Here is a close-up photo of the deck. Question 2: What care instructions will I need to follow? Is there some treatment or paint I need to apply to make it more waterproof or prevent further tearing and somehow make it more durable?

Here is another close-up photo. You can see both some chipping and some tearing, especially where knots occur in the wood. Question 3: Is this normal, what can I do to prevent it, and what did the previous owners do wrong for it to happen?

Question 4: How many summers will I be able to use it before I need to replace it? Thanks.