I'll go there if it doesn't rain.

Many times you want to say something like:

I'll go to Rock In Rio festival if it doesn't rain.

It would be instead a Soccer Game, a church service, a high school class or whatever.

It does not matter whatever thing you want to do or where you want to go or whatever else, and also doesn't matter the thing constraining or impeaching your willl.

The sentence model remains the same: I'll ________ if ________. And almost everything fits on these blanks.

Also, you can replace the I'll with the person you want to say about.

In Alethian language, the basic structure is as shown:

_______ на ________.

In other words, the needed-to-happen or the constraining reason must come in Desiderative mode (remember: the same verb forms plus the na (na) particle). This particle will also serve to "divide" the sentence accordingly.

So, let's see the translated version of that phrase?

Не амесе на Rock in Rio мацурни е юсу.

Ok, let's explain the parts:

Не амесе is the thing you need to happen. In the case, it doesn't rain. It could be another person doing something, too.

Rock in Rio мацурни е юсу is the thing you want to do.

This is the direct form of saying that. You might find it inverted as this:

Rock in Rio мацурни е юсу, не амесе на.

In case of doing this, you must add the comma (,) between the terms, and add the particle на at the very end.

VOKABULAR

ameda (v.) : to rain (conjugable at the sg. 3rd only, unless using it in connotative, i.e., in a non-dictionary, non-standardized sense, like that woman is raining in tears.) 
matsur (n) : festival, great feast
yuda (v.) : to go