I want to say "Brother will understand it," so should I use:

> あに は それ を りかい します。

Or

> あに は それ を わかります。

What other parameters am I missing?  
Is the first one preferred more and if so, why?
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grammar
りかい します Or わかります
Is this something that brother would understand, but probably not another
person? If so, I would use this expression:

> [兄]{あに}なら[分]{わ}かると[思]{おも}います。

The particle ”を” is not commonly used with the verb 分かる (though it is in rare
cases), "が" is usually more appropriate. However, it is best to omit "それが"
since it is implied.

Also, without the "と思います" part (or "と思う"), it feels very definitive to me,
like you are 100% sure. I think it's safer to add the "I think" part.

Finally, "[理解]{りかい}する" is a verb that isn't used that often in daily
conversation, though it means something similar to わかる. I have heard it most
used in the expression "理解できない！" which is when someone is complaining "I can't
even comprehend what so-and-so is doing", though there are other uses as well.
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answer
If you did something bad, let's say had an accident with your brother's car
you would say:

> 兄{あに}なら理解{りかい}してくれる  
>  or 理解{りかい}してくれるつもりだ

This is what 理解 is used for.

理解できない！ would mean "He can't understand" (why that accident happens, why all
that happened).

A second meaning is "I can't believe it" in a stronger tone than "信じれない".

So depending on what is the "it" in your question the answer will be
different.

理解{りかい} implies that you have to think, to understand something like why you
acted badly, a complicated speech/mathematics, or a complex situation.

わかる means to know something like the height of Fuji-san, the date of en event,
etc.

Both are used for different things.
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answer