Are these sentences OK?
I will let you know as soon as I decide.
I will let you know as soon as I make a decision.
I will let you know as soon as I have made a decision.

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Perfecto! And as Julie said, they increase in formality. If you "make a decision" versus "decide" it sounds like a more crucial issue that you are deciding upon, and makes the process sound more drawn-out.

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They're perfect! Good job! Now can you take those same sentences and make contractions? For example, I am going= I'm going.

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Sure, they're all fine. Probably in order of informality to formality. (first sentence least formal, last sentence most formal) but really not much difference in usage.

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