Ojalá la galería abriera un poco antes, porque por la mañana tengo clase.

No. Even though abriera looks like a past form, here it is not talking about the past.

After ojalá, Spanish often uses the imperfect subjunctive to express a wish about the present or future, especially when the wish feels unlikely or contrary to the current reality.

So Ojalá la galería abriera un poco antes means something like I wish the gallery opened a bit earlier / would open a bit earlier, not I wish the gallery opened earlier yesterday.

Because ojalá triggers the subjunctive, not the indicative.

Here is the basic contrast:

• abre = indicative, simple statement: the gallery opens
• abría = indicative, usually past habitual or descriptive: it used to open / it was opening
• abra = present subjunctive, often used for a more open or possible wish: I hope it opens
• abriera = imperfect subjunctive, often used for a more hypothetical or less realistic wish: I wish it opened / would open

In this sentence, abriera suggests the gallery does not currently open earlier, and the speaker wishes that situation were different.

Yes. Abriera and abriese are both correct imperfect subjunctive forms of abrir.

So these are both grammatical:

• Ojalá la galería abriera un poco antes
• Ojalá la galería abriese un poco antes

In modern everyday Spanish, including Spain, abriera is much more common in speech.

Because with ojalá, que is optional in many sentences.

Both of these are correct:

• Ojalá la galería abriera un poco antes
• Ojalá que la galería abriera un poco antes

There is no important change in meaning. Leaving out que is very common.

Yes, normally ojalá is followed by the subjunctive, because it expresses a wish or hope rather than a fact.

For example:

• Ojalá abra = I hope it opens
• Ojalá abriera = I wish it opened / would open
• Ojalá hubiera abierto = I wish it had opened

So if you see ojalá, you should expect a subjunctive form after it.

Because in Spanish, a singular countable noun usually needs a determiner such as el, la, un, una, mi, esta, and so on.

So galería by itself would sound incomplete here. La galería means the gallery, referring to a specific gallery that the speaker has in mind.

Un poco antes means a little earlier.

Here, antes compares the gallery's opening time with its current or usual opening time.

You often hear abrir antes when talking about schedules, opening times, arrival times, and similar ideas.

Más temprano can also work in many contexts, but abrir antes sounds especially natural here.

Compare:

• La tienda abre antes = The shop opens earlier
• Me levanté más temprano = I got up earlier

In Spain, por la mañana is the normal way to say in the morning.

The same pattern appears in:

• por la tarde
• por la noche

En la mañana does exist, but in Spain it sounds much less natural in everyday speech. It is more associated with some Latin American varieties or with more specific contexts.

Because tener clase is a very common expression meaning to have class / to have lessons / to have a class scheduled.

The forms are slightly different in meaning:

• tengo clase = I have class / I have lessons
• tengo una clase = I have one class / one specific lesson
• tengo clases = I have classes

In this sentence, tengo clase sounds very natural and general: the speaker has class in the morning.

Also, although tener clase can sometimes mean to be classy, that is clearly not the meaning here because of the school-related context.

Yes. That word order is also completely correct:

• porque por la mañana tengo clase
• porque tengo clase por la mañana

Both mean the same thing.

Putting por la mañana earlier can give it a little more emphasis, as if the speaker is highlighting in the morning as the problem.

No, it is not necessary.

You could write the sentence with or without the comma:

• Ojalá la galería abriera un poco antes, porque por la mañana tengo clase.
• Ojalá la galería abriera un poco antes porque por la mañana tengo clase.

The comma just marks a pause. In Spanish, punctuation before porque is often optional in this kind of sentence.