Focus in x Focus on x Focus at: Qual utilizar

Na frase abaixo, qual preposição seria a correta? E por que? Eu sempre fico confusa em qual preposição usar quando quero me referir a coisas no geral ou modo. I want to focus *on* grammar

Ou I want to focus *in* grammar

Ou até mesmo I want to focus *at* grammar

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2 21 119
Here's another one: "to focus in on something"

Seemingly based on photography jargon, "focus in on" = "zoom in and focus on something" has become very common in figurative language, when talking about focal points of discussion or things in need of particular attention. ... cus+in+on"
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3 24 205
We always focus on something.

You focus on grammar.
You focus on the problem.
You focus on that.
6 48 1.1k
I completely agree with NeyF on that one.

In addition to his commentary the learner might know:

If there´s a shift/change of focus, one can say that he "focus onto" (the new thing/object of attention)
At this stage of the trial the defence lawyer often shifts the focus onto the victim.
(in time - onto = onto, so focus continues being "on", the "to" part indicates the shift).

This one would mean "concentrate upon". Same thing as "focus on" (with the meaning of "to give attention, effort, etc. To one particular subject, situation or person rather than another - Oxford Learner Dict.), just in a more formal way.

You’ll soon begin to generate a picture of which ideas should be focused upon and don’t forget to refer back to your original objective about what you were trying to achieve in the first place to ensure the ideas achieve that goal.

But, using the "focus on" collocation you never will be wrong, of course.
6 48 1.1k
In time:

On technical language you can find "focus at", usually meaning "pointing at", if you use this way I think you won´t get sued. ;-)
Farsightedness (Hyperopia)

Hyperopia, unlike normal vision, occurs when the cornea is too flat in relation to the length of the eye. This causes light to focus at a point beyond the retina, resulting in blurry close vision and occasionally blurry distance vision as well. Usually this condition is undetected until later in life because the young eye is able to compensate for the hyperopia by contracting the internal lens of the eye.
It would also mean focus something (light) at a given point, literally speaking.
6 48 1.1k
While at it, there´s also: zero in on/focus attention on/center on/concentrate on/home in on/fix on/pinpoint/highlight. And informal: zoom in on.
Meaning concentrating on an aspect of something (question/issue etc...)

According to the "zero" of the Oxford American Writer´s Thesaurus.