No abras otra lata de atún hasta que lleguen mis amigas.

Because this is a negative command addressed to tú.

In Spanish, affirmative and negative tú commands use different forms:

• Affirmative: abre = open
• Negative: no abras = don’t open

So:

• Abre otra lata de atún. = Open another can of tuna.
• No abras otra lata de atún. = Don’t open another can of tuna.

The negative command uses the present subjunctive form.

The form abras tells you that.

It is the tú negative command form, so the speaker is talking to one person in an informal way.

Other possibilities would be:

• No abra = don’t open... (usted, one person, formal)
• No abráis = don’t open... (vosotros, more than one person, informal, very common in Spain)
• No abran = don’t open... (ustedes, more than one person, formal in Spain; also the normal plural form in Latin America)

Because it comes after hasta que and refers to a future event that has not happened yet.

In Spanish, after expressions like hasta que, if the action is still pending or expected in the future, you normally use the subjunctive:

• No abras otra lata de atún hasta que lleguen mis amigas.

The friends have not arrived yet, so Spanish uses lleguen.

Compare:

• Espera aquí hasta que lleguen. = Wait here until they arrive.
• Esperé aquí hasta que llegaron. = I waited here until they arrived.

In the past example, the arrival actually happened, so indicative is used: llegaron.

Use hasta before a noun, time expression, or sometimes an infinitive.

Use hasta que before a conjugated verb.

Examples:

• hasta mañana = until tomorrow
• hasta las cinco = until five o’clock
• hasta llegar a casa = until getting home / until we get home
• hasta que lleguen mis amigas = until my friends arrive

So in your sentence, que is needed because lleguen is a fully conjugated verb.

Because otro / otra normally means another by itself, without un / una.

So Spanish says:

• otra lata = another can
• otro libro = another book
• otra vez = another time / again

Using una otra would sound wrong in normal Spanish.

It literally means can/tin of tuna.

• lata = can / tin
• de atún = of tuna

Spanish often uses de in this kind of structure:

• una botella de agua = a bottle of water
• una taza de café = a cup of coffee
• una lata de atún = a can/tin of tuna

In Spain, lata is the normal word here.

Because amigas is the feminine plural form, so it refers specifically to female friends.

• mi amiga = my female friend
• mis amigas = my female friends
• mi amigo = my male friend
• mis amigos = my male friends / my friends (mixed group or masculine default)

So this sentence tells you the arriving friends are female.

Because Spanish often omits subject pronouns when the verb ending already makes the subject clear.

Here:

• abras already tells you the command is for tú
• lleguen is clarified by the noun phrase mis amigas

So Spanish does not need to say:

• Tú no abras...
• ...hasta que ellas lleguen...

Those versions are possible, but they would usually add emphasis or contrast.

Yes, a little.

The original order is very natural:

• No abras otra lata de atún hasta que lleguen mis amigas.

You can also move the hasta que clause to the front:

• Hasta que lleguen mis amigas, no abras otra lata de atún.

That is also correct and natural.

What you usually would not do is place otra lata de atún in a strange position that separates it awkwardly from abras.

Yes: atún has a written accent on the ú.

That accent shows the stress falls on the last syllable: a-TÚN.

If you wrote atun, it would be misspelled.

In Spain, if you are speaking informally to more than one person, you usually use vosotros.

So the sentence would be:

• No abráis otra lata de atún hasta que lleguen mis amigas.

That is the plural informal version used in Spain.

For comparison:

• tú: No abras...
• vosotros: No abráis...

Normally, mis amigas means my female friends, not romantic partners.

In Spanish, amiga usually means friend. If you want to make the romantic meaning clear, Spanish more often uses novia.

So:

• mis amigas = my female friends
• mis novias = my girlfriends

Without extra context, learners should understand mis amigas as my female friends.