wǒ yí jìnmén jiù kànjiàn tā le.

Because 一 changes tone depending on what comes after it.

Here, 一 comes before 进 (jìn), which is a 4th-tone syllable, so 一 is pronounced yí.

A quick rule:

• yì before a 4th tone? Actually no — the common spoken rule is:
  • 一 becomes yí before a 4th-tone syllable
  • 一 becomes yì before a 1st, 2nd, or 3rd-tone syllable
  • it stays yī when said by itself, emphasized, or used in counting

So:

• 一进门 → yí jìnmén

This is a very common pattern meaning as soon as... then... or once... immediately....

In this sentence:

• 我一进门就看见她了
• literally: I, once enter the door, then see her
• natural English: As soon as I walked in, I saw her

It shows that the second action happened immediately after the first one.

More examples:

• 我一到家就睡觉。
  As soon as I get home, I sleep / go to sleep.
• 他一听到这个消息就哭了。
  As soon as he heard the news, he cried.

In this sentence, 就 marks that the second action happens right away after the first one.

So in 一……就……:

• 一 introduces the first event
• 就 introduces the immediate result or next event

Without 就, the sentence would lose that strong immediately / as soon as feeling.

So:

• 我一进门就看见她了 = As soon as I entered, I saw her
• not just I entered, and I saw her, but the seeing happened right away

进门 literally means enter the door, but in natural English it usually means:

• come in
• enter
• walk through the door

In this sentence, it means to enter the room/house/place.

So:

• 我一进门 = as soon as I came in / as soon as I walked in

Even though it literally contains 门 (door), you usually should not translate it word-for-word as enter the door in English.

Because 看见 means to see, while 看 by itself usually means to look at.

That distinction is important:

• 看 = look, watch
• 看见 = catch sight of, see

So:

• 我看她 = I look at her
• 我看见她了 = I saw her

In your sentence, the meaning is about the result of visual perception — I saw her — so 看见 is the right verb.

The final 了 shows a new situation or change of state, and in many sentences like this it also helps signal that the event has occurred.

Here, 我一进门就看见她了 means something like:

• As soon as I came in, I saw her
• and the sentence feels complete as a reported event

Why use final 了 here?

• It marks the occurrence of the situation
• It gives the sentence a natural this happened feeling
• It often appears in narratives about completed events

It is not exactly the same as an English past tense ending. Chinese does not mark tense the same way English does.

Because the sentence is built around the pattern 一……就……, and the focus is on the immediate sequence of events, not on marking each verb separately.

So:

• 我一进门就看见她了 is the natural way to say it

If you added 了 after 进门, it would usually sound less natural in this structure, or it would change the rhythm and emphasis.

The first action (进门) is already understood as completed because it triggers the second action. The final 了 is enough to make the whole event feel completed and real in context.

It usually refers to a past event in context, but Chinese does not work like English tense.

Chinese generally relies on:

• time words
• context
• aspect markers like 了
• sentence patterns

So this sentence is naturally understood as past in many situations:

• As soon as I walked in, I saw her

But what matters grammatically is not a past-tense verb ending. Instead, Chinese shows how the action is viewed — here, as an event that happened.

Because the same subject, 我, applies to both actions:

• 进门
• 看见她

Chinese often states the subject once and then uses it for the following linked actions.

So:

• 我一进门就看见她了 means
• I entered, and I saw her immediately

You do not need to repeat 我 before 看见.

Yes. In writing, a comma is very common here because 一进门 acts like an opening time/event phrase.

So both are possible:

• 我一进门就看见她了。
• 我一进门，就看见她了。

The comma helps show a small pause:

• As soon as I came in, I saw her.

In speech, there is often a natural slight pause there too, though not always a strong one.

No. In spoken Mandarin:

• 她 = tā
• 他 = tā

They sound exactly the same.

The difference is only in writing:

• 他 = he / him
• 她 = she / her

So when listening, you usually know which one is meant from context.

Yes. That is a very good natural translation.

Depending on context, this sentence could be translated as:

• As soon as I walked in, I saw her.
• The moment I walked in, I caught sight of her.
• I saw her the instant I came in.

All of these capture the immediate feeling created by 一……就…….

Yes. The 一……就…… pattern is very flexible.

For example:

• 我一下课就回家。
  As soon as class ends, I go home.
• 他一到北京就给我打电话。
  As soon as he arrived in Beijing, he called me.
• 我一听见这个名字就想笑。
  As soon as I hear this name, I want to laugh.

So your sentence is one very typical example of a larger grammar pattern.