Η αεροσυνοδός μάς έφερε νερό και μας χαμογέλασε.

η is the feminine singular nominative article: the.

αεροσυνοδός means flight attendant. This noun can be used for a man or a woman, and the article tells you which one is meant:

• ο αεροσυνοδός = a male flight attendant
• η αεροσυνοδός = a female flight attendant

So in this sentence, the article η shows that the flight attendant is female.

This is mainly a disambiguation spelling choice.

Greek monosyllables usually do not take an accent, but some can take one when needed for clarity. Here, the first μάς helps show that it means us / to us, not our.

Without the accent, η αεροσυνοδός μας could at first be read as:

• our flight attendant

But the sentence actually means:

• the flight attendant brought us water

So μάς makes that clearer.

In the second clause, μας χαμογέλασε, the meaning is already easier to understand from context, so many writers leave it unaccented. You may still see μάς there too in careful writing.

For μας, the weak accusative and genitive forms look the same, so the form itself does not tell you.

In this sentence, it is best understood as an indirect-object-type clitic:

• μάς έφερε νερό = she brought water to us
• μας χαμογέλασε = she smiled at us / to us

Traditional grammar often treats this μας as genitive with these verbs. For a learner, the most useful thing is to recognize that μας here means the people receiving the action.

Because Greek does not always use a preposition where English does.

With χαμογελώ (to smile), Greek can use a weak pronoun like μου, σου, μας, του to show the person the smile is directed to:

• μου χαμογέλασε = she smiled at me
• μας χαμογέλασε = she smiled at us

So Greek expresses this idea more compactly than English. You should learn χαμογελώ together with this pattern.

Both are in the aorist, 3rd person singular.

In learner-friendly terms, this is the normal Greek simple past for a completed event:

• έφερε = she brought
• χαμογέλασε = she smiled

So the sentence describes two completed actions in sequence.

They are:

• έφερε ← φέρνω = to bring
• χαμογέλασε ← χαμογελώ / χαμογελάω = to smile

More specifically:

• φέρνω has the aorist έφερα, so έφερε is the 3rd person singular aorist form.
• χαμογελώ / χαμογελάω has the aorist χαμογέλασα, so χαμογέλασε is again 3rd person singular.

A useful thing to notice is that Greek past forms are often not built exactly like English past forms, so it is worth learning the aorist stem of common verbs.

Because Greek often leaves out the article with an indefinite or non-specific mass noun.

So:

• έφερε νερό = she brought water / some water

If the water were specific and already known in the conversation, Greek would more likely use the article:

• έφερε το νερό = she brought the water

So the bare νερό here sounds natural.

Because μας is a weak object pronoun (a clitic), and in normal statements Greek usually places these before the finite verb:

• μάς έφερε
• μας χαμογέλασε

This is very common in Greek. English speakers often expect the pronoun after the verb, but Greek usually does the opposite with these short clitic forms.

Yes. Greek word order is more flexible than English word order.

The sentence as written is natural and neutral:

• Η αεροσυνοδός μάς έφερε νερό και μας χαμογέλασε.

But other orders are possible if you want a different emphasis, for example:

• Μας έφερε νερό η αεροσυνοδός.

That version puts more focus on what happened to us.
What usually stays the same is the position of the clitic pronoun relative to the verb: μας normally still sits right before the finite verb.