Your son is getting bad grades in school and you want him to study harder. He complains that he can't focus on his homework and wants to know how he can become more focused. You don't think there's any special method except just trying harder. You say:
You've just got to force yourself to sit down and concentrate.
To "force" someone to do something means to make them do it:
They forced me to sit there and watch it.
As in the example at top, you can also force yourself to do something. This means to use your willpower to make yourself do something that you don't really want to do.
In English, "sitting down" is associated with doing something seriously. So "sit down and ___" means to do something carefully and deliberately:
The word "concentrate" works by itself; you can't say "concentrate (something)". If you want to express the object of someone's concentration, use "concentrate on (something):
Can you turn the TV down? I need to concentrate on this homework.
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