Αν ξεχάσω τη δεύτερη δόση, η γιατρός είπε να μην πάρω διπλή το βράδυ.

Because ξεχάσω is the aorist subjunctive, and after αν Greek often uses this form to talk about a single possible future event:

• Αν ξεχάσω = if I forget
• Αν ξεχνάω would suggest something more like if I am forgetting / if I tend to forget / if I keep forgetting, depending on context.

In this sentence, the speaker means one specific situation: if I happen to forget the second dose.

Greek normally does not use the future tense after αν in this kind of conditional clause. Instead, it uses:

• αν + subjunctive

So:

• Αν ξεχάσω... = If I forget...

Not:

• Αν θα ξεχάσω... ✘

This is very normal Greek structure. English uses if + present for future meaning (If I forget...), while Greek uses αν + subjunctive.

It is in the accusative case, because it is the direct object of ξεχάσω.

The base dictionary forms are:

• η δόση = the dose
• η δεύτερη δόση = the second dose

But as a direct object, they become:

• τη δόση
• τη δεύτερη δόση

So:

• ξεχάσω τη δεύτερη δόση = forget the second dose

Also note that τη is the feminine accusative singular form of the.

Because it agrees with δόση, which is a feminine noun.

In Greek, adjectives and ordinal numbers must match the noun in:

• gender
• number
• case

So:

• η δεύτερη δόση = feminine singular nominative
• τη δεύτερη δόση = feminine singular accusative

If the noun were masculine or neuter, the form of second would change.

Many Greek nouns ending in -ος are masculine, but γιατρός is one of those job nouns that can refer to either a man or a woman.

So you can have:

• ο γιατρός = the male doctor
• η γιατρός = the female doctor

The article tells you the gender here. Since the sentence has η γιατρός, it means the doctor is female.

Here να introduces a subordinate clause after είπε and expresses what the doctor said should happen.

So:

• η γιατρός είπε να... = the doctor said to... / said that I should...

In this sentence:

• η γιατρός είπε να μην πάρω... = the doctor said not to take...

This is a very common Greek pattern:

• είπε να φύγω = he/she said for me to leave
• μου είπε να περιμένω = he/she told me to wait

Greek often uses να + verb where English might use an infinitive (to take) or a clause (that I should take).

Because μην is the normal negative particle used with:

• subjunctive
• imperative-related meanings
• clauses with να

Since πάρω is in a να-clause, the negative must be μην:

• να μην πάρω = not to take / that I should not take

By contrast, δεν is used with ordinary indicative statements:

• δεν παίρνω το φάρμακο = I am not taking the medicine

So:

• είπε να μην πάρω ✔
• είπε να δεν πάρω ✘

πάρω is the aorist subjunctive form of παίρνω. In a να clause like this, Greek often chooses between:

• aorist subjunctive for a single complete action
• present subjunctive for ongoing, repeated, or habitual action

Here the meaning is about one act of taking a dose:

• να μην πάρω διπλή = not to take a double one

So the aorist form fits naturally.

If the meaning were more habitual or ongoing, Greek might use the present stem instead in another context.

διπλή means double. The noun δόση is omitted because it is understood from the earlier part of the sentence.

So:

• να μην πάρω διπλή literally means not to take a double [dose]

Greek often leaves out a noun when it is obvious from context, just like English can say:

• Take the red one, not the blue.

Here, διπλή is feminine singular accusative, matching the implied noun δόση.

Here το βράδυ is a time expression, meaning:

• in the evening
• at night / tonight, depending on context

It is not the object of the verb. It functions adverbially, telling you when.

Greek often uses the accusative with expressions of time:

• το πρωί = in the morning
• το μεσημέρι = at noon / in the middle of the day
• το βράδυ = in the evening / at night

So the sentence means the doctor said not to take a double dose that evening / at night.

The comma separates the if-clause from the main clause:

• Αν ξεχάσω τη δεύτερη δόση, = if I forget the second dose,
• η γιατρός είπε να μην πάρω διπλή το βράδυ. = the doctor said not to take a double one in the evening.

This works much like English punctuation when the conditional clause comes first.

Yes. Μου είπε would make the indirect object explicit:

• η γιατρός μου είπε να μην πάρω διπλή το βράδυ = the doctor told me not to take a double dose in the evening

Without μου, the sentence still makes sense, because the person being told is clear from context. Greek often leaves such things unstated when they are obvious.

It breaks down like this:

• Αν ξεχάσω τη δεύτερη δόση
  = If I forget the second dose
• η γιατρός είπε
  = the doctor said
• να μην πάρω διπλή
  = not to take a double one / not to take a double dose
• το βράδυ
  = in the evening / that night

So grammatically, it is:

• a conditional clause with αν + subjunctive
• followed by a main clause
• which contains a να-clause expressing what the doctor said should or should not be done

That combination is very common in natural Greek.