Inflamația este atât de serioasă, încât stomatologul tratează gingia.

Because inflamația is the definite form, meaning the inflammation.

In Romanian, the definite article is usually attached to the end of the noun:

• inflamație = inflammation
• inflamația = the inflammation

So the sentence talks about a specific inflammation, not inflammation in general.

Stomatologul means the dentist, while un stomatolog means a dentist.

Again, Romanian usually adds the definite article to the end of the noun:

• stomatolog = dentist
• stomatologul = the dentist

So the sentence refers to a specific dentist, or at least presents the dentist as a known participant in the situation.

Gingia is the definite form of gingie:

• gingie = gum
• gingia = the gum

So tratează gingia means treats the gum.

Romanian often uses the definite form where English might also use the, especially when referring to a specific body part in a concrete situation.

This is a very common Romanian structure meaning so ... that.

In your sentence:

• atât de serioasă = so serious
• încât = that

So:

• Inflamația este atât de serioasă, încât...
• The inflammation is so serious that...

This pattern is used to show result or consequence.

Other examples:

• Este atât de obosit, încât adoarme imediat.
  = He is so tired that he falls asleep immediately.
• Era atât de frig, încât am plecat repede.
  = It was so cold that we left quickly.

Because atât de is the normal Romanian pattern before an adjective or adverb in this kind of construction.

So Romanian says:

• atât de serios/serioasă = so serious
• atât de repede = so quickly
• atât de greu = so difficult / so hard

You should learn atât de ... încât as a set pattern.

Because serioasă must agree with inflamația, which is a feminine singular noun.

Romanian adjectives change form to match the noun:

• masculine singular: serios
• feminine singular: serioasă
• masculine plural: serioși
• feminine plural: serioase

Since inflamația is feminine singular, the adjective must also be feminine singular:

• inflamația este serioasă

Tratează is the 3rd person singular present tense of the verb a trata = to treat.

So:

• eu tratez = I treat
• tu tratezi = you treat
• el/ea tratează = he/she treats

In this sentence, the subject is stomatologul:

• stomatologul tratează = the dentist treats / is treating

Romanian present tense can often translate into English as either simple present or present progressive, depending on context.

Because Romanian usually has a postposed definite article, meaning the article is attached to the end of the noun instead of appearing before it.

Compare:

• English: the dentist

• Romanian: stomatologul

• English: the inflammation
• Romanian: inflamația

This is one of the most distinctive features of Romanian compared with other Romance languages.

The given word order is natural and standard, but Romanian word order is generally more flexible than English.

Standard version:

• Inflamația este atât de serioasă, încât stomatologul tratează gingia.

You can sometimes move elements around for emphasis, but not every change sounds equally natural. For learners, the safest choice is to keep the order:

1. subject
2. verb
3. complement/result clause

So this sentence is a good model to follow.

Încât introduces a result clause. It tells you what happens as a consequence of the first part of the sentence.

So:

• Inflamația este atât de serioasă = The inflammation is so serious
• încât stomatologul tratează gingia = that the dentist treats the gum

It connects the degree of seriousness with the result.

Yes, possibly, depending on nuance.

• serioasă = serious
• gravă = severe / serious / grave

In medical contexts, gravă can sound a bit stronger and often more specifically medical.
So:

• Inflamația este atât de gravă, încât...
  may sound more like The inflammation is so severe that...

Both can work, but gravă may be more natural in strictly medical language.

A quick guide:

• ă as in inflamația, serioasă, tratează
  This is a short central vowel, somewhat like the a in sofa.

• ț as in inflamația
  This is pronounced ts, like the end of cats.

• î as in încât
  This is a close central vowel, a sound English does not really have. It is similar to the Romanian â.

Very roughly:

• inflamația ≈ in-fla-ma-tsi-a
• încât ≈ sounds like in-kut but with a different vowel
• tratează ≈ tra-tea-ză

Because încât introduces a subordinate clause, and in Romanian it is normally separated with a comma in this kind of sentence.

So the comma helps mark the structure:

• main clause: Inflamația este atât de serioasă
• subordinate result clause: încât stomatologul tratează gingia

This punctuation is standard and important in written Romanian.