Αυτό το σύμπτωμα δεν είναι σοβαρό, αλλά θέλω να ρωτήσω τη γιατρό.

In Greek, the normal way to say this + noun is:

demonstrative + article + noun

So:

• αυτό το σύμπτωμα = this symptom

Here:

• αυτό = this (neuter singular)
• το = the (neuter singular article)
• σύμπτωμα = symptom

For an English speaker, the article may feel extra, but in Greek it is the standard pattern.
Saying αυτό σύμπτωμα would sound wrong.

A less common alternative is:

• το σύμπτωμα αυτό

That can sound more formal or emphatic.

Because σύμπτωμα is a neuter singular noun, and σοβαρό is an adjective agreeing with it.

Greek adjectives must agree with the noun they describe in:

• gender
• number
• case

So:

• σύμπτωμα = neuter singular
• σοβαρό = neuter singular form of σοβαρός / σοβαρή / σοβαρό

That is why Greek says:

• το σύμπτωμα είναι σοβαρό

not:

• σοβαρός
• σοβαρή

Those would be masculine and feminine forms.

Δεν is the normal word for negating an indicative verb in Modern Greek.

So:

• είναι = is
• δεν είναι = is not

It usually goes directly before the verb:

• δεν είναι σοβαρό

English speakers often want to place negation differently, but in Greek δεν normally comes right before the verb phrase.

Also note:

• δεν is used with ordinary statements
• μη(ν) is used in different environments, such as prohibitions or after certain particles

So here δεν is exactly the right choice.

Because Greek often leaves subject pronouns out when they are already clear from the verb ending.

• θέλω already means I want
• the -ω ending shows first person singular

So Greek does not need εγώ here.

You could say:

• Εγώ θέλω να ρωτήσω τη γιατρό

but that adds emphasis, something like:

• I want to ask the doctor
• As for me, I want to ask the doctor

In a neutral sentence, Greek usually just says θέλω.

Modern Greek does not use a normal infinitive the way English does.

Instead, it usually uses:

verb + να + finite verb

So:

• θέλω να ρωτήσω = literally something like I want that I ask
• naturally: I want to ask

This is one of the biggest structural differences from English.

Compare:

• θέλω να πάω = I want to go
• μπορώ να έρθω = I can come
• πρέπει να φύγω = I must leave

So after θέλω, Greek normally uses να plus the appropriate verb form.

Because after να, Greek often makes an aspect choice rather than a tense choice.

Here, να ρωτήσω uses the perfective / aorist stem, which presents the action as one whole act:

• θέλω να ρωτήσω τη γιατρό = I want to ask the doctor

That fits well, because the speaker means a single act of asking/consulting.

By contrast, να ρωτάω would suggest something more like:

• asking repeatedly
• asking in a habitual way
• focusing on the process rather than one complete act

So in this sentence να ρωτήσω is the natural choice.

Important: this does not make it past tense.
After να, the distinction is mainly about aspect, not past vs present.

Because the doctor is the object of the verb ρωτήσω, not the subject.

Greek changes nouns and articles depending on case.

Dictionary / subject form:

• η γιατρός = the doctor (female doctor, nominative)

Object form here:

• τη γιατρό = the doctor (accusative)

So:

• η γιατρός θέλει... = the doctor wants...
• ρωτάω τη γιατρό = I ask the doctor

For learners, the biggest visible change here is often the article:

• η → τη(ν)

And the noun appears here as:

• γιατρό

Because the final -ν of την is often dropped before many consonants in Modern Greek.

So the feminine accusative article can appear as:

• τη
• την

Before γ in γιατρό, it is very common to drop the -ν, so:

• τη γιατρό

is perfectly normal.

You may still hear or see την in some speech styles, careful pronunciation, or regional usage, but τη γιατρό is standard and very common.

You know from the article:

• τη γιατρό = female doctor
• τον γιατρό = male doctor

The noun γιατρός can be used for both male and female doctors, so the article is what tells you the gender here.

Compare:

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

And in the accusative:

• τη(ν) γιατρό = the female doctor
• τον γιατρό = the male doctor

So in this sentence, the speaker wants to ask a female doctor.

Yes. In Greek, ρωτάω / ρωτώ commonly takes the person asked as a direct object in the accusative.

So:

• ρωτάω τη γιατρό
• ρωτώ τον φίλο μου

This is very natural Greek.

English speakers sometimes expect a preposition, but Greek does not need one here.

You can also add what you are asking about, for example:

• ρωτάω τη γιατρό κάτι
• ρωτάω τη γιατρό για αυτό

So the structure in the sentence is completely normal.

Greek word order is more flexible than English word order.

The sentence as given is a very natural, neutral version:

• Αυτό το σύμπτωμα δεν είναι σοβαρό, αλλά θέλω να ρωτήσω τη γιατρό.

But Greek can move parts around for emphasis. For example:

• Δεν είναι σοβαρό αυτό το σύμπτωμα...
• Τη γιατρό θέλω να ρωτήσω.

These versions change the emphasis, not the basic meaning.

For a learner, the original sentence is the safest pattern to copy:

• subject/topic
• verb
• complement
• αλλά
• second clause

It is pronounced aftó.

The important point is the letter combination αυ. In Modern Greek, αυ is pronounced:

• av before voiced sounds and vowels
• af before voiceless sounds

Here it comes before τ, which is voiceless, so:

• αυτό = aftó

Also notice the stress is on the last syllable:

• af-TÓ