Из-за слабости и кашля терапевт сказала, что это может быть бронхит, а не обычная простуда.

Из-за means because of or due to.

It requires the genitive case, so the nouns after it change form:

• слабость → слабости
• кашель → кашля

So из-за слабости и кашля means because of weakness and cough.

A useful thing to remember: из-за often introduces a cause, and it frequently has a slightly negative flavor, which fits well here because these are symptoms.

They are different nouns, belonging to different declension patterns, so their genitive singular endings are different.

• слабость is a feminine noun ending in -ость

  • nominative: слабость
  • genitive: слабости

• кашель is a masculine noun

  • nominative: кашель
  • genitive: кашля

So both are in the same case, but they take different endings because they are different types of nouns.

Because сказала shows that the therapist is female.

The noun терапевт is grammatically shaped like a masculine noun, but it can refer to either a male or female person. In Russian, with professions like this, the verb in the past tense often tells you the person’s gender:

• терапевт сказал = the therapist was male
• терапевт сказала = the therapist was female

So even though терапевт itself looks masculine, сказала makes it clear that the speaker means a woman doctor.

Что here means that and introduces reported speech or reported information.

So:

• терапевт сказала, что... = the therapist said that...

This is extremely common in Russian. English often drops that, but Russian usually keeps что in this kind of sentence.

Мочь means can / may, and быть means to be. Together:

• может быть = may be / could be

So это может быть бронхит literally means this may be bronchitis.

This is a very normal Russian way to express a possible diagnosis, identification, or explanation.

Good question. After быть, Russian can use either the nominative or the instrumental, depending on the structure and style.

In sentences with это meaning this/it is, the nominative is very common:

• Это бронхит
• Это может быть бронхит

So бронхит stays in the nominative here.

Using the instrumental in this exact structure would sound less natural. For identification with это, nominative is the usual choice.

Because it matches бронхит in the comparison:

• это может быть бронхит, а не обычная простуда

Both бронхит and простуда are functioning as predicate nouns after это может быть, and here Russian uses the nominative.

Also, простуда is feminine, so the adjective must agree with it:

• обычный → masculine
• обычная → feminine

So:

• обычная простуда = an ordinary/common cold

А не is very common when contrasting one possibility with another:

• бронхит, а не обычная простуда
  = bronchitis, not an ordinary cold

Here а sets up a contrast: this, rather than that.

Compare the feel:

• а не = contrast/replacement
• но не = more like but not

In this sentence, а не is the more natural choice because the doctor is distinguishing between two diagnoses.

Usually yes, in everyday contexts.

Простуда is the normal word for a cold or what English speakers casually call a cold. In everyday speech it can sometimes be a bit broader than a precise medical term, but in this sentence обычная простуда clearly means an ordinary/common cold as opposed to something more serious like bronchitis.

Russian has no articles, so there is no direct equivalent of a, an, or the.

That means:

• бронхит can mean bronchitis
• простуда can mean a cold or the cold, depending on context

English needs articles; Russian does not. The exact meaning is understood from the situation.

Yes. Russian word order is more flexible than English word order, because case endings do a lot of the grammatical work.

This sentence uses a very natural order:

• Из-за слабости и кашля терапевт сказала, что это может быть бронхит, а не обычная простуда.

That order presents:

1. the reason/symptoms,
2. the speaker,
3. what was said.

You could rearrange parts for emphasis, but the original sounds neutral and natural.

Placing it first gives the background immediately: the symptoms are the reason for the doctor’s conclusion.

Russian often puts context-setting information early in the sentence. Starting with из-за слабости и кашля helps frame everything that follows: because of these symptoms, the therapist said it might be bronchitis.

So this is a natural information flow, not a special grammatical rule.

Here it means may be as a verb phrase:

• это может быть бронхит = it may be bronchitis

That is different from the adverb может быть meaning maybe/perhaps in other contexts.

Compare:

• Это может быть бронхит. = It may be bronchitis.
• Может быть, это бронхит. = Maybe it’s bronchitis.

Same words, but different structure and function.

Терапевт is in the nominative case because it is the subject of сказала.

So:

• терапевт сказала = the therapist said

Even though other parts of the sentence involve other cases, the subject stays in the nominative.