Desde que uso hilo dental y enjuague cada noche, la dentista dice que mi boca está mejor.

Yes, it is the time meaning of since: from the time that or ever since.

So Desde que uso hilo dental y enjuague cada noche... means that this habit started earlier and is still true now.

Important distinction:

• desde que = since in a time sense
• porque = because
• ya que / puesto que = since in a reason/cause sense

So in this sentence, desde que is not giving a reason. It is marking the starting point of an ongoing situation.

Spanish often uses the present tense for an action that started in the past and still continues now, especially with expressions like desde que.

So:

• Desde que uso hilo dental... = Since I started using dental floss... / Ever since I’ve been using dental floss...

English often prefers a present perfect or present perfect continuous here, but Spanish does not need that structure.

This is very common:

• Vivo aquí desde 2020 = I’ve lived here since 2020
• Desde que trabajo aquí, estoy más tranquila = Since I’ve been working here, I’m calmer

Because Spanish usually omits subject pronouns when they are not needed.

The ending -o in uso already tells you the subject is I.

So:

• Desde que uso hilo dental... = natural
• Desde que yo uso hilo dental... = possible, but more emphatic or contrastive

You would add yo only if you want to stress it, compare yourself with someone else, or avoid ambiguity.

In Spanish, when you talk about using something in a general habitual way, it is often natural to leave out the article.

So:

• uso hilo dental = I use dental floss / I floss
• uso enjuague = I use mouthwash

Using el is possible, but it can sound more like you mean a specific floss or a specific mouthwash.

Compare:

• Uso hilo dental cada noche = general habit
• Uso el hilo dental que me recomendó la dentista = that specific floss

So the article-free version fits the idea of a regular routine.

Here it is a noun.

It means something like mouthwash or rinse, probably short for enjuague bucal.

Why we know it is a noun:

• It follows uso
• It is coordinated with another noun phrase: hilo dental y enjuague

So the structure is basically:

• uso [hilo dental] y [enjuague]

There is also a verb form enjuague, but that would be a subjunctive or command form, and it does not fit here.

A useful note for Spain: many speakers would find enjuague bucal or colutorio a bit more explicit or idiomatic than just enjuague, though enjuague is understandable.

Yes, both are possible and both usually mean every night.

• cada noche = every night
• todas las noches = every night / all nights

The difference is small. Cada noche can feel slightly more like repeating the habit one night at a time, while todas las noches simply states the frequency very directly.

In this sentence, either works well.

Because Spanish normally uses que to introduce a reported statement.

• La dentista dice que mi boca está mejor
• literally: The dentist says that my mouth is better

In English, that is often optional:

• The dentist says my mouth is better
• The dentist says that my mouth is better

In Spanish, que is usually required here. Leaving it out would sound wrong.

Dentista is a common-gender noun. That means the noun form stays the same, and the article shows whether the person is male or female.

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

So in your sentence, la dentista tells you the dentist is a woman.

That is a great question. Spanish often does use the definite article with body parts, but usually when the owner is already clear from a reflexive or indirect object structure.

For example:

• Me duele la boca = My mouth hurts
• Me lavo los dientes = I brush my teeth

But in your sentence, the structure is different:

• la dentista dice que mi boca está mejor

Here, using mi is natural because you are clearly identifying whose mouth the dentist is talking about. Without mi, la boca could sound less personal or less clearly anchored.

So the body-part rule is real, but it does not apply in exactly the same way in every sentence.

Because estar is used for a current state or condition, and the sentence is talking about your mouth’s health as it is now.

• mi boca está mejor = my mouth is better / in better condition
• mi boca es mejor would sound odd here, as if your mouth were inherently superior in some general way

With health, recovery, and improvement, Spanish very often uses estar:

• Estoy mejor
• La herida está mejor
• Tus dientes están mejor

So está mejor is exactly what you would expect for an improved condition.

Because the sentence starts with a subordinate clause:

• Desde que uso hilo dental y enjuague cada noche, ...

When that kind of introductory clause comes first, Spanish normally separates it from the main clause with a comma.

So the comma helps show the structure:

• introductory time clause
• main clause

If the order were reversed, the comma would usually disappear:

• La dentista dice que mi boca está mejor desde que uso hilo dental y enjuague cada noche.

It is understandable, but some speakers in Spain might prefer a slightly different wording.

Two especially natural alternatives would be:

• Desde que uso hilo dental y enjuague bucal cada noche, la dentista dice que mi boca está mejor.
• Desde que uso hilo dental y colutorio cada noche, la dentista dice que mi boca está mejor.

A speaker might also say:

• Desde que uso hilo dental y enjuague bucal todas las noches, la dentista dice que tengo la boca mejor.

That said, the original sentence is grammatically fine, and the main thing a learner should notice is the structure with desde que, the present tense, and está mejor.