Σήμερα έχει τόση λιακάδα που, αν δεν κάτσω λίγο στη σκιά, καίγομαι γρήγορα.

This is a very common Greek pattern: έχει + noun can mean there is / the weather is ... in an impersonal way.

So:

• έχει λιακάδα = it’s sunny / there’s bright sunshine
• έχει ζέστη = it’s hot
• έχει κρύο = it’s cold
• έχει αέρα = it’s windy

Here, έχει does not really mean that some specific subject has something. It works more like an impersonal weather expression.

Λιακάδα means bright sunshine, sunny weather, or strong sun.

It is more vivid than just saying ήλιος (sun). In this sentence, λιακάδα suggests not just that the sun is out, but that it is strongly sunny and bright.

So τόση λιακάδα gives the feeling of so much blazing sunshine.

Τόση ... που is a very common result construction in Greek. It means:

• so much / so ... that

In this sentence:

• τόση λιακάδα που ... = so much sunshine that ...

This is parallel to English structures like:

• It’s so sunny that I burn quickly
• He’s so tired that he fell asleep

A few similar Greek examples:

• Έχει τόση ζέστη που δεν αντέχεται.
  = It’s so hot that it’s unbearable.

• Μιλάει τόσο γρήγορα που δεν τον καταλαβαίνω.
  = He speaks so fast that I can’t understand him.

Notice:

• τόση agrees with λιακάδα, which is feminine singular.
• With a masculine noun, you would get τόσος
• With a neuter noun, τόσο
• As an adverb, τόσο

Because λιακάδα is a feminine singular noun, and τόσος / τόση / τόσο behaves like an adjective here, so it agrees with the noun.

Forms:

• masculine singular: τόσος
• feminine singular: τόση
• neuter singular: τόσο

So:

• τόση λιακάδα
• τόση ζέστη
• τόσο φως
• τόσος αέρας

If you said τόσο λιακάδα, that would be ungrammatical.

Here που means that, introducing the result clause:

• τόση λιακάδα που ...
• so much sunshine that ...

Greek που has several uses, and this is one of them. It is not really a relative pronoun here. It is a connector introducing the consequence/result.

So in this sentence:

• main idea: Σήμερα έχει τόση λιακάδα
• result: που ... καίγομαι γρήγορα

In other words: Today there’s so much sunshine that ... I burn quickly.

After αν when Greek talks about a possible future condition, it usually uses the subjunctive-type form without να, often with the perfective stem.

So:

• αν δεν κάτσω = if I don’t sit / if I don’t go sit
• not if I am not sitting in a continuous sense

This is the normal pattern for a real future condition in Modern Greek.

Compare:

• Αν φάω, θα νιώσω καλύτερα.
  = If I eat, I’ll feel better.

• Αν δεν φύγεις τώρα, θα αργήσεις.
  = If you don’t leave now, you’ll be late.

So κάτσω here is natural because the meaning is if I don’t take the action of sitting for a bit in the shade.

If you used κάθομαι, it would sound more like a habitual or ongoing-state idea, and it would not fit this structure as naturally.

Yes. Κάτσω is the form used here for the verb meaning sit down / sit / stay seated, and it is related to κάθομαι.

Very roughly:

• κάθομαι = I sit / I am sitting / I stay
• κάτσω = form used for to sit down / to sit (as a single action) in contexts like αν, να, θα

So in this sentence:

• αν δεν κάτσω λίγο στη σκιά
  = if I don’t sit for a bit in the shade

This is a very common everyday form.

Yes. Καθίσω is also possible.

In everyday speech, many speakers prefer κάτσω, which sounds very natural and colloquial. Καθίσω is also correct and common, sometimes sounding a bit more neutral or slightly more formal depending on context and speaker.

So both of these are possible:

• αν δεν κάτσω λίγο στη σκιά
• αν δεν καθίσω λίγο στη σκιά

For a learner, it is useful to recognize both.

Because after αν in this kind of conditional sentence, Greek normally does not use να.

Compare:

• Θέλω να κάτσω.
  = I want to sit.

but

• Αν κάτσω, θα νιώσω καλύτερα.
  = If I sit, I’ll feel better.

So the form may look similar to a subjunctive form, but after αν, να is omitted.

That is a very important pattern in Greek:

• να φύγω = that I leave / to leave
• αν φύγω = if I leave

Here λίγο means for a little while / for a bit.

So:

• να κάτσω λίγο = to sit for a bit

It is functioning adverbially here, modifying the verb rather than describing a noun.

Compare:

• λίγο νερό = a little water
  Here it modifies a noun.

• κάτσε λίγο = sit for a bit / wait a moment
  Here it modifies the action.

In your sentence, the idea is not sit only a small amount of shade, but sit briefly in the shade.

Στη is the contraction of:

• σε + τη → στη

So:

• στη σκιά = in the shade

This is very common in Greek:

• στο σπίτι = σε + το σπίτι = at home / in the house
• στη θάλασσα = σε + τη θάλασσα = in the sea / to the sea
• στον ήλιο = σε + τον ήλιο = in the sun

The article is used because Greek normally says in the shade with the definite article, just as English usually does.

Greek often uses the present tense for something that is generally true, immediate, vivid, or regularly happens under those conditions.

So:

• καίγομαι γρήγορα literally = I burn quickly / I get burned quickly

In natural English, depending on the context, you might say:

• I burn quickly
• I get burned quickly
• I’ll burn quickly

Greek present tense here works well because the speaker is describing a real, immediate consequence of the situation: whenever it’s this sunny, if I don’t sit in the shade, I burn quickly.

It has a very natural general truth / immediate reality feel.

Καίγομαι is the middle/passive-form verb of καίω (burn), but in actual usage it often means:

• I get burned
• I am burning
• I burn

In this sentence, it most naturally means I get burned / I get sunburned quickly.

Greek often uses middle/passive forms in ways that do not match English passive structure exactly. So it is better to learn καίγομαι here as the normal verb for I get burned.

Because Greek is a pro-drop language: the verb ending usually shows the subject clearly enough.

Here:

• κάτσω = I sit / I sit down in this form
• καίγομαι = I get burned

So Greek does not need to say εγώ unless the speaker wants emphasis or contrast.

For example:

• αν δεν κάτσω λίγο στη σκιά, καίγομαι γρήγορα
  = normal, neutral

• αν δεν κάτσω λίγο στη σκιά, εγώ καίγομαι γρήγορα
  = more emphatic, like I’m the one who burns quickly

The conditional clause is inserted into the middle of the result clause, so commas help separate it clearly.

The structure is basically:

• Σήμερα έχει τόση λιακάδα που ... καίγομαι γρήγορα.

Inside that, the speaker inserts the condition:

• αν δεν κάτσω λίγο στη σκιά

So the commas mark it off as an inserted subordinate clause.

Without commas, the sentence would be harder to read.

A helpful breakdown is:

• Σήμερα = today
• έχει = there is / it has
• τόση = so much
• λιακάδα = sunshine / sunny weather
• που = that / so that
• αν = if
• δεν = not
• κάτσω = I sit / I sit down
• λίγο = for a bit
• στη σκιά = in the shade
• καίγομαι = I get burned
• γρήγορα = quickly

That shows how Greek builds the thought grammatically, even if the most natural English wording may be a little different.

Not naturally, if the idea is specifically I get burned by the sun.

Καίγομαι is the normal and direct choice for I get burned. In context, it strongly suggests burning from sun exposure.

A Greek speaker could express the idea in other ways, but they would not be as simple or direct. For example, they might say something like παθαίνω έγκαυμα in a different context, but for everyday sun exposure, καίγομαι is exactly the kind of verb you would expect.

Yes, very much so.

It sounds natural because it uses several very everyday features of spoken Greek:

• έχει λιακάδα
• τόση ... που
• αν δεν κάτσω
• στη σκιά
• καίγομαι γρήγορα

A native speaker would easily say something like this in ordinary conversation, especially in summer or when talking about sensitivity to the sun.