Το ότι ιδρώνεις τόσο εύκολα είναι φυσιολογικό όταν έχει τόση ζέστη.

Το ότι is a very common Greek way to turn a whole clause into the fact that ...

So:

• ιδρώνεις τόσο εύκολα = you sweat so easily
• το ότι ιδρώνεις τόσο εύκολα = the fact that you sweat so easily

In this sentence, that whole clause becomes the subject of είναι φυσιολογικό:

• Το ότι ιδρώνεις τόσο εύκολα είναι φυσιολογικό
  = The fact that you sweat so easily is normal

Greek often uses το ότι where English uses that or the fact that.

Ιδρώνεις is the 2nd person singular present of ιδρώνω = to sweat.

So it means:

• ιδρώνω = I sweat
• ιδρώνεις = you sweat
• ιδρώνει = he/she/it sweats

The sentence is talking directly to one person, so Greek uses ιδρώνεις = you sweat.

Τόσο εύκολα means so easily.

Breakdown:

• τόσο = so / that much
• εύκολα = easily

Here εύκολα is an adverb, not an adjective.
Adverbs do not change to agree with gender, number, or case.

Compare:

• εύκολος / εύκολη / εύκολο = easy (adjective)
• εύκολα = easily (adverb)

So:

• ιδρώνεις εύκολα = you sweat easily
• ιδρώνεις τόσο εύκολα = you sweat so easily

Because the subject of the sentence is the whole clause το ότι ιδρώνεις τόσο εύκολα, and Greek treats that as neuter singular.

So the predicate adjective also appears in the neuter singular form:

• φυσιολογικός = masculine
• φυσιολογική = feminine
• φυσιολογικό = neuter

That is why Greek says:

• Το ότι ιδρώνεις τόσο εύκολα είναι φυσιολογικό

Literally, it works like:

• That you sweat so easily is normal

The whole fact/clause is being described as normal, so Greek uses the neuter form.

It literally means when it has so much heat, but natural English would be when it is so hot.

Breakdown:

• όταν = when
• έχει = it has / there is
• τόση ζέστη = so much heat

Greek very often uses έχει ζέστη to mean it is hot.

So:

• έχει ζέστη = it is hot
• έχει πολύ ζέστη = it is very hot
• έχει τόση ζέστη = it is so hot / there is so much heat

This is a very idiomatic Greek weather expression.

Because in Greek, weather and environmental conditions are often expressed with έχει rather than είναι.

For example:

• έχει ζέστη = it is hot
• έχει κρύο = it is cold
• έχει ήλιο = it is sunny
• έχει αέρα = it is windy

Using είναι ζέστη would sound wrong in standard Greek.
So this is something to learn as a fixed expression: έχει ζέστη.

Because ζέστη is a feminine noun.

So τόσος / τόση / τόσο has to agree with the noun it describes:

• τόσος = masculine
• τόση = feminine
• τόσο = neuter

Since ζέστη is feminine, Greek uses:

• τόση ζέστη = so much heat / such heat

This is agreement just like with adjectives and many other words in Greek.

No. These are different words.

• ότι = that
• ό,τι = whatever / anything that

In your sentence, ότι is the conjunction meaning that:

• Το ότι ιδρώνεις τόσο εύκολα... = The fact that you sweat so easily...

By contrast:

• Πάρε ό,τι θέλεις = Take whatever you want

The comma in ό,τι matters.

Sometimes Greek can use just ότι, but in this kind of structure το ότι is very common because it clearly means the fact that ...

Compare:

• Το ότι ιδρώνεις τόσο εύκολα είναι φυσιολογικό
  = The fact that you sweat so easily is normal

Without το, the sentence would be less natural in this specific pattern.
So for learners, it is safest to treat το ότι + clause as a standard expression meaning the fact that ...

Yes, Greek can sometimes use το να + verb to make a verbal idea into a noun-like phrase:

• Το να ιδρώνεις τόσο εύκολα είναι φυσιολογικό

This is also possible and means something like:

• Sweating so easily is normal
• To sweat so easily is normal

The difference is subtle:

• το ότι ... focuses more on the fact that
• το να ... focuses more on the action/process

In your sentence, το ότι sounds very natural because the speaker is commenting on the fact itself.

The sentence has two main parts:

1. Το ότι ιδρώνεις τόσο εύκολα
   = the subject
   = the fact that you sweat so easily

2. είναι φυσιολογικό όταν έχει τόση ζέστη
   = the predicate
   = is normal when it is so hot

So the overall structure is:

• [Subject clause] + είναι + [adjective] + όταν-clause

More literally:

• The fact that you sweat so easily is normal when there is so much heat.

Greek word order is fairly flexible, but the original order is the most straightforward and natural.

Standard order:

• Το ότι ιδρώνεις τόσο εύκολα είναι φυσιολογικό όταν έχει τόση ζέστη.

You might move things around for emphasis, for example:

• Είναι φυσιολογικό να ιδρώνεις τόσο εύκολα όταν έχει τόση ζέστη.

That version is also very natural, but it is slightly restructured.

So yes, Greek can change word order, but the original sentence is clear, neutral, and idiomatic.