Se vi mensogas al mi, mi tuj rimarkas tion.

Because -as is the present tense, and in Esperanto the present tense is also used for general truths, habits, and repeated situations.

So this sentence does not have to mean only one specific moment right now. It can mean:

If/whenever you lie to me, I notice it immediately.

That is why mensogas and rimarkas both sound natural here.

Because -us is for something hypothetical, less real, or contrary to fact.

• Se vi mensogas al mi, mi tuj rimarkas tion = a real or general condition
• Se vi mensogus al mi, mi tuj rimarkus tion = If you were to lie to me, I would notice it immediately

So se does not automatically require -us. You use -us only when the meaning is hypothetical.

Because the person you lie to is introduced with al.

In Esperanto, mensogi works like lie, and the listener is not a direct object here. So:

• mensogi al iu = to lie to someone

That is why al mi is correct, and min would be wrong in this sentence.

Al usually shows direction or a recipient, and here it marks the person receiving the lie.

So vi mensogas al mi is literally something like:

you lie to me

This is very normal Esperanto usage with verbs of speaking or communicating.

Tuj means immediately, right away, or at once.

So mi tuj rimarkas tion means:

I notice that immediately.

It adds the idea that there is no delay.

Because tion is the accusative form of tio.

• tio = that
• tion = that as a direct object

Here, rimarkas takes a direct object: you notice something. The thing being noticed is that fact, so Esperanto uses tion.

It refers to the whole previous idea: the fact that you are lying to me.

Esperanto often uses tio/tion to point back to an entire clause or situation. So here tion means something like:

that fact or that

In English, we often do the same thing: I notice that.

Yes, sometimes you can, if the context already makes it clear.

• Mi tuj rimarkas tion = I notice that immediately
• Mi tuj rimarkas = I notice immediately

The version with tion is a little more explicit, because it clearly says what is being noticed.

Yes. Esperanto word order is fairly flexible because endings show the grammar.

For example, you could also say:

Mi tuj rimarkas tion, se vi mensogas al mi.

That still means the same thing. The original order is very natural because it gives the condition first and then the result.

It is standard and natural to use a comma here because Se vi mensogas al mi is a subordinate clause.

So the comma separates:

• the if-clause
• the main clause

That makes the sentence easier to read.

Either. Esperanto vi is used for:

• singular you
• plural you
• formal you
• informal you

So only the context tells you whether one person or several people are meant.

Yes. Esperanto allows that:

Mi tuj rimarkas tion, se vi mensogas al mi.

This is grammatically fine. Putting Se vi mensogas al mi first is just a very common and natural way to present the condition before the result.