[Acuarey]

First, sorry for the bad english
Second, sorry for the bad Chemestry(Still in 1st year)

I´d like to know what "settles" a compound as "stable" or not
By "stable" I mean, a compound which cannot easily react. For instance, Na2CO3 + MgSO4 --> Na2SO4 + MgCO3  (As far as I am concern, that is not a "valid" reaction). Another exemple, I was told it is hard to react with NaCl, but why? Because its Ionic?
If yes, then why when I mix a Covalent(Acid) with a Ionic(Hydroxide) I get Water(Covalent)? What makes a reaction possible or not?

And, btw, which are the "most" stable?: H2SO4, NaCl, CO2, H2O, AgCl, HCl, Na2CO3, NaHCO3.

Thank you for all the *delete me*
Sorry for being stupid, if something is not clear please ask me!

[Arkcon]

First, sorry for the bad english
Second, sorry for the bad Chemestry(Still in 1st year)

Don't worry about either of those, we're glad to help.

I´d like to know what "settles" a compound as "stable" or not
By "stable" I mean, a compound which cannot easily react.

Too many things to get into in a forum position, but we'll talk about a few things to start you off with.

For instance, Na2CO3 + MgSO4 --> Na2SO4 + MgCO3  (As far as I am concern, that is not a "valid" reaction).

I have no idea what you mean here.  That is a perfectly reasonable reaction.

Another exemple, I was told it is hard to react with NaCl, but why? Because its Ionic?

I have no idea what you mean here, NaCl undergoes a number of reactions.  In fact, this Wikipedia page here actually refutes your two examples: https://en.wikipedia.org/wiki/Magnesium_carbonate#Preparation

If yes, then why when I mix a Covalent(Acid) with a Ionic(Hydroxide) I get Water(Covalent)? What makes a reaction possible or not?

And, btw, which are the "most" stable?: H2SO4, NaCl, CO2, H2O, AgCl, HCl, Na2CO3, NaHCO3.

These will be easier to answer when you correct or clarify your initial positions.

Thank you for all the *delete me*
Sorry for being stupid, if something is not clear please ask me!

[Acuarey]

First, thx for being so comprehensive!
Now, let me reformulate everything

When we heat up(Add energy) a compound there are(as far as I am concern) three "most likely" results:
1-It just changes its state of matter. Solid becomes liquid. Water does that on a daily basis. H2O(s) --> H2O(l)
2-It reacts chemecally with another compound. A stoven usually works this way in my country. CH4 + 2O2 ---> CO2 + 2H2O
3-It decomposes it self into "more stable" compounds. The bubles in Coca-cola. H2CO3 ----> H2O + CO2

By stable, I mean a compound that won´t react (Helium).
Or that won´t react with a large number of compounds(As you showed as an exemple those reactions involving salt(NaCl). Even though it is in the chemichal reaction, it is a Product, not a Reagent.
Or that is going to demand a huge amount of energy(Heat, maybe) to decompose(CaCO3---> CaO + CO2).

Some compounds are more "reactive" than others, because they can work as Reagents with many compounds.
So I ask again: Which are the most stable compounds? And why are they so "Stable"? Why will they have a "hard time" reacting with another compound?

[Corribus]

So I ask again: Which are the most stable compounds? And why are they so "Stable"? Why will they have a "hard time" reacting with another compound?

One of the most important aspects of chemistry (and one that you'll hear many times if you continue in the field) is that there is an interplay between thermodynamics and kinetics, and both of these play a role in reactivity (and stability). They are related but distinct concepts, and both feed into probably core idea of equilibrium, which is central to understanding the way that chemicals interact. In a sense you can think of a chemical system in terms of two dimensions or coordinates: energy and time. Thermodynamics relates primarily to the first, kinetics to the second. As a simple way to think about these two concepts, consider a simply system of a glass of hot tea being held in your (comparatively cold) hand. You know from experience that heat will flow spontaneously from the hot water into your skin. This is thermodynamics - literally, the change of heat - which governs where energy goes. An important thing to realize is that chemistry is about change. Nothing is absolutely stable or unstable - chemicals are only stable or unstable compared to something else. If we compare two states of a system, the thermodynamically stable system is the one that represents the "final" position of where energy would like to be. In the case of you holding a cup of hot water, the initial state is "unstable" in that we know it will spontaneously change into another state (colder water, hotter hand). When the cup, hand, and environment have all reached the same final temperature, such that no more spontaneous change happens - this is a thermodynamically stable system, until something else comes along to make it unstable again. Also from experience, you know that if the hot beverage is in a Styrofoam cup versus a ceramic mug, the rate at which the thermodynamically unstable system changes to the stable system can be quite different: you can hold a hot cup of coffee in a good insulated container for quite a long time, but if you did that with a ceramic mug, which does not insulate as well, your hand would burn. Note that in the end, the final state will be the same (the coffee WILL cool) but the kinetics of the process are different. In fact, if the cup was insulated well enough, you may not even notice a change at all, and the thermodynamically unstable system could actually seem practically stable. We would call such a system thermodynamically unstable but kinetically stable. Stability therefore depends not only on where energy is likely to go, but also your frame of reference of time - the coffee in a thermos system is kinetically stable on a timescale of minutes or even hours, but not on a timescale of years. In chemistry, there are lots of examples of thermodynamically unstable but kinetically stable. One that you might be familiar with is gasoline in oxygen. By themselves, nothing happen, despite the fact that you know these two substances can react vigorously if given a spark. This system is thermodynamically very unstable but kinetically very stable. Much of understanding why reactions occur boils down to knowing what the "Styrofoam" is that causes some reactions to occur slowly and other quickly, despite it being favorable for energy to flow from one form to another. 

This isn't something one can condense into a single post - this is what chemistry is all about.

[Acuarey]

I see, it´s all about Entropy. I am asking which "energy" will spread "faster", but that is not what the Second Law of Thermodynamics says. It doesn´t say when energy will spred, it says that whenever it can, It will.

Thx fr the relativity exemple. It all depends on the "mug" Its being used.
I fell really stupid now XD. It was like I gave you: x + y = 5, and I was asking which was bigger.
But "this is what chemistry is about"- A nice quote.

Thx for all the help guys!