После падения врач сначала думал, что это вывих, но позже исключил его и посоветовал беречь ногу.

Because the preposition после always takes the genitive.

• после падения = after the fall
• dictionary form: падение
• genitive singular: падения

This is a very common pattern:

• после урока = after the lesson
• после работы = after work
• после аварии = after the accident

So после падения is grammatically required, not optional.

Сначала means at first or initially.

In this sentence:

• врач сначала думал = the doctor initially thought

It shows that this was the doctor's first idea, which was later changed.

A common contrast is:

• сначала = at first
• позже / потом = later / afterwards

So the sentence has a clear timeline:

1. first, the doctor thought it was a dislocation
2. later, he ruled that out
3. then he advised protecting the leg

In the present tense, Russian usually omits the verb to be.

So:

• это вывих literally looks like this dislocation
• but it means it is a dislocation

This is completely normal in Russian:

• Это врач. = This is a doctor.
• Он дома. = He is at home.
• Это проблема. = It is a problem.

If the sentence were in the past or future, Russian would use forms of быть:

• Это был вывих. = It was a dislocation.
• Это будет вывих? = Will it be a dislocation?

Because after это, Russian often uses the predicate noun in the nominative case.

So:

• это вывих = it is a dislocation

This is a very standard pattern.

You may also see instrumental in other kinds of sentences, especially with past/future быть or with some stylistic differences, but here это + nominative is the normal choice.

For a learner, the safest rule is:

• Это X → usually nominative
• Это врач
• Это проблема
• Это вывих

Вывих means a dislocation.

It refers to an injury where a joint has been displaced.

English speakers sometimes confuse it with:

• перелом = fracture / broken bone
• ушиб = bruise / contusion
• растяжение = sprain / strain
• вывих = dislocation

So the doctor first thought the injury was a dislocation.

Because его here refers back to вывих, which is a masculine noun.

• вывих is masculine
• accusative singular pronoun referring to it: его

So:

• исключил его = ruled it out

Even though его can also mean him, the context makes it clear that it refers to the possible diagnosis, not to a male person.

Russian pronouns often work like this:

• перелом → исключил его
• травма → исключил её
• повреждение → исключил его

The pronoun agrees with the grammatical gender of the noun it replaces.

In medical Russian, исключить often means to rule out.

So:

• позже исключил его = later he ruled it out

This is a very natural use in Russian, especially when talking about diagnoses.

Examples:

• Врач исключил перелом. = The doctor ruled out a fracture.
• Нужно исключить инфекцию. = We need to rule out an infection.

So although the basic idea of исключить is exclude, in this context the best English translation is rule out.

Исключил is perfective past, which presents the action as completed.

Here that makes sense, because the doctor reached a conclusion:

• first he thought one thing
• later he ruled it out completely

If you used исключал, it would sound more like an ongoing process, repeated action, or background activity, which does not fit as well here.

So:

• исключил = ruled out, completed the decision
• исключал = was ruling out / used to rule out / ruled out repeatedly

Because думал focuses on a state of thinking or an opinion he had for some time: he thought.

In this sentence, the idea is not he had a brief thought, but rather:

• at first, the doctor believed / thought that it was a dislocation

That is why the imperfective думал works well.

Compare:

• Он думал, что это вывих. = He thought it was a dislocation.
• Он подумал, что это вывих. = He had the thought that it was a dislocation / it occurred to him that it was a dislocation.

The second version sounds more like a single mental event; the first sounds more natural here.

Because in Russian, just like in English, the subject can be omitted when it is clearly the same.

So:

• врач сначала думал
• но позже исключил его
• и посоветовал беречь ногу

All three verbs still refer to the doctor.

English does the same:

• The doctor first thought..., but later ruled it out and advised...

Repeating врач every time would be possible, but less natural.

This is a common Russian pattern:

• посоветовать + infinitive = to advise someone to do something

So:

• посоветовал беречь ногу = advised [someone] to protect / take care of the leg

The infinitive беречь expresses what was advised.

More examples:

• Врач посоветовал больше отдыхать. = The doctor advised resting more.
• Она посоветовала обратиться к специалисту. = She advised seeing a specialist.

Russian often does not explicitly mention the person receiving the advice if it is obvious from context.

Because the advice is about an ongoing, repeated, careful behavior, not one single completed action.

• беречь ногу = protect the leg / take care of the leg

The imperfective infinitive is natural for general advice, recommendations, habits, and processes.

Compare:

• Врач посоветовал беречь ногу. = The doctor advised taking care of the leg.
• Врач посоветовал отдохнуть. = The doctor advised getting some rest.

The first is ongoing care; the second can be understood as a more bounded action.

Because беречь takes a direct object, and direct objects are usually in the accusative case.

• dictionary form: нога
• accusative singular: ногу

So:

• беречь ногу = to protect the leg

Other examples:

• беречь здоровье = protect your health
• беречь спину = take care of your back
• беречь глаза = protect your eyes

Russian often leaves out possessive words like his, her, or one's when the meaning is obvious.

So:

• беречь ногу naturally means take care of the leg, and in context that means the injured leg

If needed for emphasis or clarity, Russian can add:

• беречь свою ногу
• беречь больную ногу

But in ordinary speech, the shorter version is very natural.

English often prefers an explicit possessive, while Russian often does not.