Question: Which one is appropiate?

> 
> I will submit my assignments by tonight
> 
> 
> I will be submitting my assignments by tonight.
> 
> 
> 

I want to express that before the end of the day, I will submit my assignments.

Answer: 
First off, with the preposition *by*, we would normally mention a specific time or point in time. I will submit my assignments by 10 tonight or by midnight tonight. Or you can say *by the end of the day*, because that also refers to a specific point in time. (*By tonight* does not really specify a point in time, unless there is a previously defined specific time that *tonight* refers to, such as a deadline that everyone knows about.)

As far as which construction to use, either one is fine. The first one has more of a promise meaning. The second one has a more matter of fact meaning. I explain the difference in detail [here](https://ell.stackexchange.com/a/48043/6951)
In this case, "I will submit" is the correct usage.

"I will submit" focuses on the fact that there will be an event, where you submit the assignment. The listener can reasonably assume it will be swift.

"I will be submitting" focuses on the act of you engaging in submission. There will be a submission, but it concentrates on the time period where the submission is occurring, rather than the fact that it happens. I would use the phrasing "I will be submitting my assignments by tonight" if the listener knows I intend to submit my assignment in the form of an interpretive dance in the cafeteria drawn out over 3 acts, and I will be intentionally disrupting normal class to do so.

This is more obvious in other verbs. "I will pray for you" may be as little as "I'll pray for you once, but then I'm going about the rest of my business." "I will be praying for you implies a longer duration and many prayers. (*though some people will say "I will pray for you" and pray many times over a long duration. Welcome to English. We're not as consistent as we'd like.*)