Could you explain it me? Which tense is used?
Thanks in advance
It is simple past. (I don't think this is considered the present progressive/present continuous tense. I could be wrong, though.)
If you do something over and over again, we use the expression to keep + an -ing word.
He keeps eating, but he doesn't get fat.
I have trouble doing algebra problems, but I keep trying.
Present tense of your example would be:
They keep trying until they win.
Your example is the simple past:
They kept trying until they won.
By the way, there is another construction that means the same thing:
I try and try, because I know I will eventually succeed. = I keep trying, because I know I will eventually succeed.
He eats and eats, but he doesn't get fat. = He keeps eating, but he doesn't get fat.
Tempus is correct... and Damian is also correct
"Keep" is normally used with a gerund to mean that you continue doing an action.
She kept talking.
We kept walking.
They kept stealing.
Were you studying when she called?
You were not studying when she called.
it is of course Pimple Past tense. And as for this -ing ending, in this case, this word is a gerund (a verb which functions as a noun).
Thanks to everyone !