Когда медсестра готовила укол, я отвернулся, потому что не люблю смотреть на шприц.

Because готовила is imperfective, and here it describes an action that was in progress in the background:

• Когда медсестра готовила укол... = When the nurse was preparing the injection...

The speaker’s action, я отвернулся, happened during that process.

If you used приготовила, it would usually mean the preparation was completed:

• Когда медсестра приготовила укол... = When the nurse had prepared the injection...

That changes the timing slightly. In your sentence, the point is that while she was preparing it, the speaker turned away.

Укол literally means something like a prick or a shot, but in medical contexts it usually means an injection.

So:

• делать укол = to give an injection
• готовить укол = to prepare an injection

It does not mean the syringe itself. That is шприц.

So the sentence contains both:

• укол = the injection / shot
• шприц = the syringe

Because отвернуться is a reflexive verb meaning to turn away.

• отвернуть = to turn something away / to turn something
• отвернуться = to turn oneself away

So:

• я отвернулся = I turned away

The -ся form is important here because the subject is changing his own direction.

In the past tense, Russian verbs agree with the subject’s gender.

• я отвернулся = spoken by a man
• я отвернулась = spoken by a woman

So this sentence assumes the speaker is male. If the speaker were female, it would be:

• Когда медсестра готовила укол, я отвернулась, потому что не люблю смотреть на шприц.

This is a very common Russian pattern:

• imperfective for the background, ongoing action
• perfective for the single completed action that happens within that situation

Here:

• медсестра готовила укол = the nurse was preparing the injection
• я отвернулся = I turned away

So the sentence presents:

1. an ongoing process
2. a one-time reaction

That contrast is very natural in Russian.

Because не люблю смотреть на шприц expresses a general fact about the speaker, not just a one-time past feeling.

It means:

• because I don’t like looking at syringes

This is similar to English:

• I turned away because I don’t like needles.

Even though the event happened in the past, the dislike is still presented as a current/general characteristic.

If you said не любил, that would sound more like a past-only state:

• because I didn’t like looking at syringes

That is possible in some contexts, but не люблю is more natural for a general personal preference or aversion.

Because the verb смотреть normally takes на + accusative when it means to look at something.

So:

• смотреть на шприц = to look at the syringe
• смотреть на врача = to look at the doctor
• смотреть на море = to look at the sea

This is just the standard pattern of the verb.

Compare:

• видеть шприц = to see the syringe
• смотреть на шприц = to look at the syringe

In other words, видеть is about perception, while смотреть на is about directing your gaze.

Because шприц is a masculine inanimate noun, and for masculine inanimate nouns, the accusative singular is usually the same as the nominative singular.

So:

• nominative: шприц
• accusative: шприц

That is why after на you still see шприц, even though на here requires the accusative.

Compare with an animate masculine noun:

• я вижу врача = I see the doctor

There the accusative changes form because врач is animate.

They refer to two different things:

• укол = the injection / shot itself
• шприц = the syringe

So the nurse is preparing the injection, and the speaker says he does not like looking at the syringe.

This distinction is important, because English learners may sometimes treat both as just shot or needle, but Russian separates the medical procedure from the instrument.

Because Когда медсестра готовила укол is a subordinate clause introduced by когда (when).

Russian normally separates subordinate clauses from the main clause with a comma:

• Когда медсестра готовила укол, я отвернулся...

That is the same basic idea as in English:

• When the nurse was preparing the injection, I turned away...

Russian punctuation is quite strict here, so the comma is required.

Yes. Russian word order is flexible, although the original version is very natural.

For example, you could say:

• Я отвернулся, когда медсестра готовила укол, потому что не люблю смотреть на шприц.

This is also grammatical.

The original order is good because it sets the scene first:

• Когда медсестра готовила укол... = background
• я отвернулся = main event

Russian often uses word order to control emphasis and flow rather than basic grammatical roles, since case endings already show those roles clearly.