मैंने सुबह खिड़की खोली, लेकिन मेरे भाई ने दरवाज़ा नहीं खोला।

Because Hindi often uses ने with the subject in the perfective past when the verb is transitive (that is, it takes a direct object).

Here, खिड़की खोली means opened the window, and the action is completed, so Hindi uses:

• मैंने = I
  • ergative marker ने

Compare:

• मैं खिड़की खोलता हूँ = I open / I am opening the window
• मैंने खिड़की खोली = I opened the window

So ने is a very common marker in past completed actions with transitive verbs.

For the same reason: भाई is also the subject of a completed transitive action.

In the second clause:

• मेरे भाई ने दरवाज़ा नहीं खोला
• My brother did not open the door

Since open is a transitive verb and the action is in the perfective past, ने is used again with the subject.

Each clause has its own subject and verb, so each clause needs its own grammar.

Because the verb agrees with the direct object here, not with the subject.

Since the subjects have ने, the perfective verb usually agrees with the direct object if that object has no को.

So:

• खिड़की is feminine
• therefore: खोली is feminine singular

And:

• दरवाज़ा is masculine
• therefore: खोला is masculine singular

So the pattern is:

• खिड़की खोली = opened the window
• दरवाज़ा खोला = opened the door

This is one of the most important things to notice in Hindi past-tense sentences with ने.

Because भाई is followed by the postposition ने, and that makes the noun phrase take the oblique form.

With masculine singular possessives like मेरा, the form changes before an oblique noun:

• मेरा भाई = my brother
• मेरे भाई ने = my brother + ने

So मेरा changes to मेरे because the whole phrase is in the oblique form before ने.

Some nouns change visibly in the oblique, and some do not.

भाई is one of those nouns whose direct and oblique singular forms look the same:

• direct: भाई
• oblique: भाई

So the change is visible in मेरे, not in भाई itself.

That is why you see:

• मेरा भाई
• मेरे भाई ने

In Hindi, नहीं usually comes before the verb or before the part being negated.

So:

• दरवाज़ा नहीं खोला = did not open the door

This is the normal placement in simple sentences.

Compare:

• मैं जाता हूँ = I go
• मैं नहीं जाता हूँ = I do not go

In your sentence, नहीं directly negates खोला.

Hindi does not have articles like a, an, and the.

So:

• खिड़की can mean a window or the window
• दरवाज़ा can mean a door or the door

The exact meaning depends on context.

English must choose between a and the, but Hindi usually leaves that unstated unless something else makes it clear.

Because these are direct objects, and in this sentence they are left unmarked.

In Hindi, को is often used with:

• animate direct objects
• specific or affected objects in some contexts
• indirect objects

But with inanimate objects like खिड़की and दरवाज़ा, it is very common to leave them without को.

That is important here, because when the object is unmarked, the perfective verb can agree with it:

• खिड़की खोली
• दरवाज़ा खोला

If को were added, the agreement pattern would change.

Hindi often uses time words directly, without a preposition or postposition.

So:

• सुबह = in the morning / morning
• शाम = in the evening
• आज = today
• कल = yesterday / tomorrow

In this sentence:

• मैंने सुबह खिड़की खोली
• I opened the window in the morning

English needs in the, but Hindi often does not.

No, Hindi word order is fairly flexible, though there are natural patterns.

The neutral order here is:

• मैंने सुबह खिड़की खोली
• subject + time + object + verb

And then:

• लेकिन मेरे भाई ने दरवाज़ा नहीं खोला

Because Hindi is generally SOV: subject-object-verb.

Still, you could move सुबह for emphasis:

• सुबह मैंने खिड़की खोली
• In the morning, I opened the window

Both are grammatical; the difference is mostly emphasis and style.

लेकिन means but and joins two contrasting ideas.

So the sentence contrasts:

• I opened the window in the morning
• but my brother did not open the door

It works much like English but.

Other common Hindi contrast words include:

• पर = but
• मगर = but

लेकिन is very common and neutral.

Yes. The dictionary form is खोलना, meaning to open.

Its perfective forms change for gender and number:

• खोला = masculine singular
• खोली = feminine singular
• खोले = masculine plural
• खोलीं = feminine plural

In your sentence:

• खिड़की खोली because खिड़की is feminine
• दरवाज़ा खोला because दरवाज़ा is masculine

So these are the completed-action forms of खोलना.

Because the subject changes.

The first clause is about I:

• मैंने ... खोली

The second clause is about my brother:

• मेरे भाई ने ... नहीं खोला

So Hindi, like English, normally states the new subject clearly when it changes. If you left it out, the sentence would become confusing or sound incomplete.

This repetition is natural and necessary here.