Learn Hardcore Japanese: watasi ha byouin de tyuusya wo mite sukosi kowakatta desu. - When I saw the injection at the hospital, I felt a little scared.

Using 私は is natural because the focus is on the experience and feeling, not on who was scared (that’s obvious in context).
If you used 私が, it would sound more contrastive, like:

私が病院で注射を見て少し怖かったです。
It was *I (as opposed to someone else) who was a bit scared when I saw the injection at the hospital.*

That nuance is usually not needed here, so 私は is standard.

Why is で used after 病院 instead of に?

で marks the place where an action happens.
Here, both actions 注射を見て (saw the injection) and 少し怖かった (felt a bit scared) happened at the hospital, so 病院で is correct.

Rough guide:

場所＋で: place where an action or event occurs
- 病院で待ちました。 – I waited at the hospital.
場所＋に: destination or existence
- 病院に行きます。 – I will go to the hospital.
- 病院に先生がいます。 – There is a doctor at the hospital.

In your sentence, 病院 is not a destination; it’s the setting of what happened, so 病院で is used.

What exactly does 注射 mean here? Is it the injection, the act, or the needle?

注射 can refer to:

the act of giving an injection,
the shot as a medical treatment,
or, in everyday speech, effectively the needle / injection setup you see.

In 注射を見て少し怖かった, it naturally means:

When I saw the injection (i.e., the shot / needle), I felt a bit scared.

Native speakers will understand it as the thing you are about to get stuck with, not just an abstract action.

Why is 注射 marked with を? Can 見る use が here?

見る is a transitive verb that normally takes を:

映画を見る – to watch a movie
景色を見る – to look at the scenery

So 注射を見る is the standard pattern: object＋を＋見る.

Using 注射が見える is also possible but different:

注射を見る – you actively look at the injection.
注射が見える – the injection is visible / can be seen.

In your sentence, you are actively seeing the injection, so 注射を見て with を is the natural choice.

What is the role of the て-form in 見て here?

The て-form connects clauses. Here, 見て links the action seeing the injection to the result feeling scared.

Common nuances of the て-form:

Sequence: did A and then B
Cause/trigger: when A happened, B (resulted)

In 注射を見て少し怖かったです, the nuance is:

When I saw the injection, I felt a bit scared.
or
I saw the injection and (as a result) felt a bit scared.

So 見て functions like a soft “and then / when”, often implying a cause.

Why is 怖かった in the past tense while 見て is not? How do I know the overall tense?

In Japanese, the final main predicate (here 怖かったです) usually controls the tense for the whole sentence.

見て is in て-form, which is tenseless by itself.
怖かったです is the past form of 怖い, so the sentence describes a past experience.

Interpretation:

注射を見て – having seen the injection / when I saw the injection
少し怖かったです – I was a little scared

You infer that the seeing also happened in the past because it’s connected to a past feeling. If you changed it to present, you would say:

注射を見て少し怖いです。 – When I see the injection, I feel a bit scared. (general/present habit)

Why do we need です after 怖かった? Isn’t 怖かった already complete?

Grammatically, 怖かった alone is a complete sentence:

少し怖かった。 – I was a little scared.

However, adding です makes it polite. With い-adjectives:

Present: 怖いです (polite) vs 怖い (plain)
Past: 怖かったです (polite) vs 怖かった (plain)

So 少し怖かったです is the polite past:
I was a little scared (said politely).

What nuance does 少し add? Is there a difference between 少し and ちょっと here?

少し means a little / a bit and sounds slightly more neutral or formal than ちょっと.

In this sentence:

少し怖かったです。 – I was a little scared. (neutral/polite)

If you say:

ちょっと怖かったです。

the meaning is very similar, but ちょっと is:

more casual / conversational, and
sometimes used as a softener (like kind of / a bit).

Both are fine here; 少し fits well in a textbook-style, polite sentence.

Could I drop 私は and still be correct? How would that change the feeling?

Yes, you can absolutely drop 私は:

病院で注射を見て少し怖かったです。

Japanese often omits the subject when it’s clear from context. In a conversation about your experience, listeners will naturally assume I as the subject.

Nuance:

私は…: explicitly sets me as the topic (maybe emphasizing my experience).
(no 私): more natural in everyday speech; less explicitly focused on “as for me”, but usually understood as “I” anyway.

Both are grammatically correct; the version without 私 is very common in real conversation.

Can I change the word order, like 私は少し怖かったです、病院で注射を見て?

That word order sounds unnatural in Japanese.

Basic flow in your sentence:

Set the topic: 私は
Give the setting/action: 病院で注射を見て
Give the result/feeling: 少し怖かったです

Japanese word order is somewhat flexible, but:

Clause order (cause → result) is important.
Time and place usually come before the main verb/feeling.
A postposed 病院で注射を見て after the main clause feels off or incomplete.

Natural variations:

私は病院で注射を見て、少し怖かったです。
病院で注射を見て、少し怖かったです。 (dropping 私は)

But moving 病院で注射を見て to the very end is not natural here.

Does this mean “I was a little scary” or “I was a little scared”? How do I tell?

It means “I was a little scared”, not “scary”.

The adjective 怖い describes how frightening something is to someone, but in Japanese:

私は怖い。 literally is “I am scary (to others).”
私は犬が怖い。 is “I am scared of dogs.” (lit. “Dogs are scary (to me).”)

In your sentence, because the context is seeing an injection (which is normally scary), it’s clear that:

the injection is scary, and
you are the one feeling fear.

So 少し怖かったです is understood as “I felt a bit scared (by it)”, not “I was a bit frightening.” The causal 見て also pushes the meaning toward “I felt scared when I saw it.”

How would I say “I was very scared” instead? Do I just replace 少し with とても?

Yes, you can replace the degree word:

病院で注射を見てとても怖かったです。
– I was very scared when I saw the injection at the hospital.

Other common options:

すごく怖かったです。 – I was really scared. (more casual)
本当に怖かったです。 – I was truly / really scared.

The structure stays the same; you just change 少し to another adverb that indicates degree of fear.