Die Tür quietscht, wenn ich sie am Morgen öffne.

Because Tür is a feminine noun in German. Its dictionary form is die Tür.

In this sentence, die Tür is also the subject of the main clause, so it is in the nominative case. For a feminine noun, the nominative article is still die.

Because Tür is feminine, the matching pronoun is sie.

Here, sie refers back to die Tür:

• Die Tür = the door
• ich öffne sie = I open it

In English, you would say it, but German uses grammatical gender, so a feminine noun is replaced by sie.

Because sie is the accusative pronoun here, and öffnen normally takes a direct object.

So:

• ich öffne sie = I open it

For a feminine singular pronoun:

• nominative: sie
• accusative: sie
• dative: ihr

You would only use ihr if the verb required the dative, which öffnen does not.

This is because the sentence has:

• a main clause: Die Tür quietscht
• a subordinate clause introduced by wenn: wenn ich sie am Morgen öffne

In German:

• In a main clause, the conjugated verb usually comes in second position
• In a subordinate clause with wenn, the conjugated verb goes to the end

So:

• Die Tür quietscht
• wenn ich sie am Morgen öffne

That final verb position is one of the most important German word order rules.

Here, wenn means something like when or whenever.

Because the sentence describes a repeated or typical situation, English might naturally translate it as:

• when I open it in the morning
• whenever I open it in the morning

So wenn often introduces:

• repeated actions
• general conditions
• things that happen whenever something else happens

German often uses the present tense for things that happen regularly or are generally true.

This sentence describes a habitual situation:

• the door squeaks
• when I open it in the morning

So the present tense is completely natural:

• quietscht
• öffne

English does the same:

• The door squeaks when I open it in the morning

Am is a contraction of an dem.

So:

• an dem Morgen → am Morgen

In time expressions, am is very common:

• am Morgen = in the morning
• am Abend = in the evening
• am Montag = on Monday

Literally, am Morgen is closer to at the morning or on the morning, but the natural English translation is in the morning.

Yes. Morgens is also very common.

Compare:

• am Morgen = in the morning, often referring to the morning as a time period
• morgens = in the mornings, mornings, regularly in the morning

So you could say:

• Die Tür quietscht, wenn ich sie morgens öffne.

That version can sound even more clearly habitual. Both are natural.

Because in German, a subordinate clause is normally separated from the main clause with a comma.

Since wenn ich sie am Morgen öffne is a subordinate clause, the comma is required:

• Die Tür quietscht, wenn ich sie am Morgen öffne.

German punctuation is stricter than English here.

Yes. You can also put the wenn clause first:

Wenn ich sie am Morgen öffne, quietscht die Tür.

That is still correct, but notice what happens in the main clause after the subordinate clause:

• the conjugated verb comes immediately after the subordinate clause

So you get:

• Wenn ich sie am Morgen öffne, quietscht die Tür.

Not:

• Wenn ich sie am Morgen öffne, die Tür quietscht.

Because ich is the subject of the wenn clause.

In:

• wenn ich sie am Morgen öffne

the subject is ich, and the object is sie.

So it means:

• when I open it in the morning

Meanwhile, in the main clause:

• Die Tür quietscht

the subject is die Tür.

So the sentence has two different subjects in two different clauses:

• die Tür squeaks
• ich open it

It comes from the verb quietschen, which means to squeak.

The verb is conjugated here for er/sie/es because the subject is die Tür:

• ich quietsche
• du quietschst
• er/sie/es quietscht

So:

• Die Tür quietscht = the door squeaks

Because quietschen does not need an object here. The door itself is doing the action.

So:

• Die Tür quietscht = The door squeaks

But in the second clause, öffnen does need an object:

• ich öffne sie = I open it

So the pronoun sie appears only where it is grammatically needed.

A rough pronunciation guide:

• quietscht sounds approximately like kveetcht
• öffne sounds roughly like UF-nuh, but with the German ö sound, which does not exist exactly in English

A few helpful details:

• qu in German is usually pronounced kv
• tsch sounds like ch in church
• ö is a rounded front vowel; to practice it, try saying eh while rounding your lips

So:

• quietscht = kvietscht
• öffne = öff-ne

Most likely it describes a general habit or repeated situation.

That is because:

• wenn often introduces repeated situations
• the present tense suggests something regular
• am Morgen can describe a typical time when this happens

So the sentence usually feels like:

• The door squeaks whenever I open it in the morning

If you wanted to make it sound more like one specific occasion, German would often use a different structure or more context.