La dentista recomienda usar hilo dental y un enjuague después de cenar.

Dentista already has the same form for both masculine and feminine. Many Spanish profession nouns ending in -ista work like this.

• el dentista = the male dentist
• la dentista = the female dentist

So in this sentence, la tells you the dentist is female.

Both are possible, but they are used a little differently.

• La dentista recomienda usar... = The dentist recommends using...
• La dentista recomienda que uses... = The dentist recommends that you use...

In your sentence, usar is an infinitive, which is common when giving general advice or when the person doing the action is understood from context.

If Spanish wants to make the second person more explicit, it often uses que + subjunctive:

• La dentista recomienda que uses hilo dental...

So the version in the sentence sounds like a general recommendation.

Spanish often leaves out the article with objects when speaking in a general way, especially after verbs like usar.

So:

• usar hilo dental = to use dental floss, to floss

This sounds natural and general, like talking about the habit itself.

You could sometimes hear usar el hilo dental in a more specific context, for example if you are referring to a particular floss already mentioned. But here the article is not needed.

This is because the two nouns are being treated slightly differently.

• hilo dental is often used as an uncountable or generic item in this kind of advice.
• un enjuague is being used as a countable noun, meaning something like a rinse or a mouthwash.

In dental context, un enjuague is short for something like un enjuague bucal.

So the structure feels natural:

• usar hilo dental = use dental floss
• usar un enjuague = use a mouthwash / do a rinse

Here, enjuague means rinse or, more naturally in this context, mouthwash.

On its own, enjuague can refer to the act of rinsing or to a rinse product. In a dental sentence like this, most learners should understand it as:

• enjuague bucal = mouthwash

In Spain, you may also hear colutorio for mouthwash.

Because después normally needs de before a noun or infinitive.

So you say:

• después de cenar = after having dinner
• después de la cena = after dinner / after the dinner meal

The de is required here.

They are very close in meaning, but the structure is different.

• después de cenar = after eating dinner / after having dinner
• después de la cena = after dinner / after the evening meal

The first uses an infinitive, so it focuses on the action of dining.
The second uses a noun, so it names the meal itself.

In everyday use, both can work in many situations.

Yes, cenar means to have dinner / to eat the evening meal.

In Spain:

• desayunar = to have breakfast
• comer = to have lunch / eat
• cenar = to have dinner

Because meal times in Spain are often later than in some English-speaking countries, cenar may happen later in the evening than a learner expects, but the basic meaning is still to have dinner.

No. In standard Spanish, h is silent.

So hilo is pronounced roughly like EE-lo.

That means:

• hilo dental sounds like EE-lo den-TAL

Yes. Both refer to a dentist, but dentista is the more common everyday word.

• dentista = dentist
• odontólogo / odontóloga = dentist, more formal or technical

So La dentista recomienda... sounds very natural in normal conversation.

Spanish word order is flexible, but the most neutral order is:

• subject + verb + rest of the sentence

So:

• La dentista recomienda usar hilo dental...

That is the most straightforward way to say it.

A different order, such as Recomienda la dentista..., is possible in some contexts, but it sounds more marked and is not the neutral default a learner should copy first.