Если болит колено, лучше не нагружать его.

Если means if. It introduces a condition:

Если болит колено, лучше не нагружать его.
= If your knee hurts, it’s better not to strain it.

A learner might expect when, but Russian uses если because this is a conditional statement: in the situation where the knee hurts, this is the best thing to do.

Russian often leaves possession unstated when it is obvious from context, especially with body parts.

So болит колено literally looks like the knee hurts, but in natural English we usually say your knee hurts or the knee hurts depending on context.

If you want to make the owner explicit, you can say:

Если у тебя болит колено, лучше не нагружать его.
= If your knee hurts, it’s better not to strain it.

Without у тебя / у вас / у меня, the sentence sounds more general, like medical advice.

Because колено is the grammatical subject of болит.

In Russian, the thing that hurts is often the subject:

• Колено болит. = The knee hurts.
• Голова болит. = The head hurts / I have a headache.

So колено stays in the nominative.

English often structures this differently, but Russian treats the body part itself as the subject.

Because the subject колено is singular.

• колено = knee
• болит = hurts / aches for one thing

Compare:

• Болит колено. = The knee hurts.
• Болят колени. = The knees hurt.

So the verb agrees with the noun in number.

Болит means hurts, aches, or is painful.

It comes from болеть. With body parts, it is very commonly used for pain:

• болит спина = the back hurts
• болит рука = the arm hurts
• болит колено = the knee hurts

So here it does not mean being sick in a general sense. It specifically refers to pain in the knee.

This is a common Russian pattern:

лучше + infinitive
= it’s better to ...

So:

лучше не нагружать его
= it’s better not to strain it

Russian often leaves out an explicit it is in this kind of sentence. English needs it’s better, but Russian can simply say лучше.

Other examples:

• Лучше подождать. = It’s better to wait.
• Лучше не спешить. = It’s better not to hurry.

Because this sentence gives general advice, not a direct command.

• лучше не нагружать его = it’s better not to strain it
• не нагружай его = don’t strain it

The version with лучше + infinitive sounds softer and more neutral. It is like advice from a doctor or health guide, rather than someone directly ordering you.

Нагружать is the imperfective form, and it fits well here because the sentence is about general, ongoing advice: don’t put stress on it / don’t load it.

Imperfective is natural when talking about:

• repeated actions
• ongoing actions
• general recommendations

Here the meaning is not don’t complete one single act of loading, but rather avoid stressing the knee in general.

A perfective form would sound less natural in this medical-advice context.

Literally, нагружать means to load or to put a load on. In this context, with a body part, it means:

• to strain
• to put stress on
• to overwork

So не нагружать его means not to put too much stress on the knee.

This is a very natural way to talk about injuries, joints, muscles, and recovery.

Because его is the accusative form of the pronoun, used for a direct object.

• оно = it as a subject
• его = it as an object

Here, the knee is the thing being strained:

нагружать его = to strain it

Since нагружать takes a direct object, Russian uses его.

Because for animate and inanimate things, Russian personal pronouns still have gender.

• колено is neuter
• nominative pronoun: оно = it
• accusative pronoun: его = it

So:

• Оно болит. = It hurts.
• Не нагружай его. = Don’t strain it.

This is completely regular for neuter nouns.

Yes. Russian word order is flexible.

Both of these are natural:

• лучше не нагружать его
• лучше его не нагружать

The meaning is basically the same. The second version puts его a little earlier, which can make the object slightly more prominent.

The original sentence is very natural and neutral.

It is general advice. It means something like:

If a knee hurts, it’s better not to strain it.

In actual use, it can also apply to one particular person if the context makes that clear. But by itself, the sentence sounds like general medical advice rather than a statement about one named person.

To make it personal, Russian would often add something like:

• Если у тебя болит колено... = If your knee hurts...
• Если у вас болит колено... = If your knee hurts... (formal/plural)

Yes, you could.

• Лучше не нагружать его = It’s better not to strain it
• Лучше не нагружать колено = It’s better not to strain the knee

Using его avoids repeating колено, so it sounds smoother. Russian often replaces a repeated noun with a pronoun just like English does.