Beside, besides and aside

Beside

A palavra beside é uma preposuição que  significa ao lado (perto ou proximo de algo ou  alguém)
The word beside is a preposition. It means close to or next to.

Examples:

Come and sit beside me.

Your hat is beside the dog basket.

By my side. your side (estar ao lado de alguém no sentido de companhia)

> I wish you were by my side.
I will be always by your side. 

Besides

 A preposição besides significa em adição a ou a parte disso.   Como advérbio significa além  disso, além de ou aliás.
The preposition besides means in addition to or apart from.  As an adverb, it means furthermore or and another thing.

Examples:

Besides Craig, who else caught a bass?
(Besides is a preposition in this example. Apart from Craig, who else...)

Besides, it's not just about determination.
(Besides is an adverb in this example. Furthermore, it's not just...)


http://www.grammar-monster.com/easily_confused/beside_besides.htm


 Comparação entre besides, beside e aside


1) You can sit beside me.
2)  We can go out next week, besides i don't have money enough.
2a) Besides Ann, who else is going with me??
3) I will put my fears aside and face the risks!

Resumo:

Beside: ao lado, proximo ou perto de (preposição)
Besides: além disso, aliás
Aside: por algo de lado ou a parte

Now, study and practise this text in english about this lesson and try to undertand the context on your own.

What is the difference between aside and beside?

Answer:

Besides indicates something additional to what has already been stated. Besides often suggests that the addition is in the nature of an afterthought: The bill cannot be paid as yet; besides, the work is not completed.

be⋅sides 
-adverb
1. moreover; furthermore; also: Besides, I promised her we would come.
2. in addition: There are three elm trees and two maples besides.
3. otherwise; else: They had a roof over their heads but not much besides.

-preposition
4. over and above; in addition to: Besides a mother he has a sister to support.
5. other than; except: There's no one here besides Bill and me.


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a·side
adv.
To or toward the side: step aside.
Out of one's thoughts or mind: put my doubts aside.
Apart: a day set aside for relaxing.
In reserve; away: put a little money aside.
Set out of the way; dispensed with: All joking aside, can you swim 15 miles?
n.
A piece of dialogue intended for the audience and supposedly not heard by the other actors on stage.
A remark made in an undertone so as to be inaudible to others nearby.
A parenthetical departure; a digression.

http://wiki.answers.com/Q/What_is_the_difference_between_aside_and_beside