Врач объяснил, что бронхит часто начинается постепенно, а ангина, наоборот, обычно начинается резко.

Because it agrees with the subject врач (doctor), which is grammatically masculine here. In the past tense, Russian verbs change for gender and number:

• объяснил = masculine singular
• объяснила = feminine singular
• объяснило = neuter singular
• объяснили = plural

So Врач объяснил means The doctor explained, with the doctor understood as male or grammatically masculine.

Because что introduces a subordinate clause. Russian normally puts a comma before что in sentences like this:

• Он сказал, что... = He said that...
• Я знаю, что... = I know that...
• Врач объяснил, что... = The doctor explained that...

So the comma marks the beginning of the clause being explained.

Because they are the subjects of the verb начинается.

In each part of the sentence:

• бронхит начинается = bronchitis begins
• ангина начинается = angina / tonsillitis begins

The thing that begins is the subject, so Russian uses the nominative case.

Because the verb here is начинаться, not начинать.

• начинать = to begin something, to start something
  Example: Он начинает урок = He starts the lesson
• начинаться = to begin, to start
  Example: Урок начинается = The lesson begins

Diseases are treated as things that begin on their own, so Russian uses начинаться:

• бронхит начинается
• ангина начинается

The -ся ending often gives this intransitive or middle meaning.

Because Russian often uses the present tense for general truths, regular patterns, or typical behavior.

Here the sentence means something like:

• Bronchitis often begins gradually
• Angina usually begins suddenly

This is not about one specific event happening right now. It is a general medical observation, so the present tense is natural.

Because the sentence describes a usual or repeated pattern, not one completed event.

Начинаться is imperfective. Imperfective verbs are commonly used for:

• general facts
• repeated actions
• habitual situations
• processes

Since the doctor is describing how these illnesses typically begin, the imperfective is the right choice.

Here а marks contrast. It is often somewhere between and and but in English.

The idea is:

• bronchitis often begins gradually, whereas angina usually begins suddenly

So а does not simply join two equal facts like и would. It sets up a comparison or contrast between them.

Наоборот means on the contrary, conversely, or by contrast, depending on context.

In this sentence it strengthens the contrast:

• бронхит часто начинается постепенно
• а ангина, наоборот, обычно начинается резко

So it highlights that the second illness tends to begin in the opposite way from the first.

Because it functions as a parenthetical/discourse word here rather than as a core part of the sentence structure.

Russian often uses commas around words like this when they comment on the whole statement:

• наоборот
• конечно
• например
• поэтому (sometimes depending on structure)

So ангина, наоборот, обычно начинается резко means roughly angina, by contrast, usually begins suddenly.

Both are adverbs, but they are not exactly the same.

• часто = often
  Focuses on frequency
• обычно = usually
  Focuses on what is typical or normal

So:

• бронхит часто начинается постепенно = it often happens this way
• ангина обычно начинается резко = this is the usual pattern

In many contexts they are close, but not identical.

They are adverbs modifying начинается.

• постепенно = gradually
• резко = sharply, suddenly, abruptly

They describe how the illness begins.

So:

• начинается постепенно = begins gradually
• начинается резко = begins suddenly/abruptly

Russian often uses резко in medical or descriptive language to mean that symptoms appear abruptly or sharply.

A learner may expect внезапно, which also means suddenly, and that word could work in some contexts. But резко is very natural when describing a sudden, sharp onset, especially in more formal or medical-style statements.

So начинается резко sounds natural in this context.

Usually, Russian keeps it here for clarity and balance.

The full parallel structure is:

• бронхит часто начинается постепенно
• ангина, наоборот, обычно начинается резко

You could sometimes omit repeated material in Russian if it is very obvious, but here repeating начинается sounds more natural and makes the contrast neat and explicit.

Yes, Russian word order is flexible, but the original order is natural and clear.

The sentence puts the disease first, then the frequency word, then the verb, then the manner adverb:

• бронхит часто начинается постепенно
• ангина, наоборот, обычно начинается резко

This order sounds neutral and informative. Other orders are possible for emphasis, for example:

• Бронхит начинается часто постепенно — less natural in this context
• Постепенно бронхит часто начинается — awkward
• Обычно ангина начинается резко — possible, with a slightly different emphasis

So the original version is a good standard model.

Because Russian has no articles.

Whether something is understood as a doctor, the doctor, bronchitis, or the sore throat/angina depends on context, word order, and meaning.

So Russian simply says:

• Врач = doctor / the doctor / a doctor
• бронхит = bronchitis
• ангина = angina / tonsillitis / sore throat depending on context

Not usually. This is a very important false friend.

In Russian, ангина usually means tonsillitis or a severe sore throat, especially in everyday usage.

In English, angina usually refers to chest pain related to heart disease.

So learners should be careful not to assume the English medical meaning when they see Russian ангина.

That is just their grammatical gender:

• бронхит is masculine
• ангина is feminine

You can often guess gender from the ending:

• words ending in a consonant are often masculine: бронхит
• words ending in -а / -я are often feminine: ангина

This matters for agreement in other contexts, though in this sentence the verb начинается is present tense, so it does not show gender.

Only the past-tense verb would change:

• Врач объяснил... = male doctor explained
• Врач объяснила... = female doctor explained

The rest of the sentence would stay the same:

• Врач объяснила, что бронхит часто начинается постепенно, а ангина, наоборот, обычно начинается резко.