Siempre uso hilo dental después de cenar.

Because Spanish often drops subject pronouns when the verb ending already shows who the subject is.

• uso = I use
• The -o ending tells you it is first person singular.

So Siempre uso hilo dental... already means I always use dental floss...
You could say Yo siempre uso hilo dental..., but yo is usually only added for emphasis, contrast, or clarity.

Uso is the present simple, and in Spanish the present simple is very commonly used for habits and routine actions.

So:

• Siempre uso hilo dental después de cenar = I always use dental floss after dinner / I always floss after dinner

If you said estoy usando, that would usually mean I am using it right now, which does not fit a general habit as well.

Hilo dental means dental floss.

Literally:

• hilo = thread
• dental = dental

So the literal idea is dental thread. In everyday use, it simply means dental floss.

In this sentence, hilo dental is being used in a general, non-specific way, so no article is very natural.

• Uso hilo dental = I use dental floss / I floss
• Uso el hilo dental = more like I use the dental floss or I use the floss, referring to a more specific floss

So the version without the article is the most natural for a general habit.

Because después normally needs de before a noun or an infinitive.

So:

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

But if después stands alone, it can mean afterwards:

• Ceno y después uso hilo dental. = I have dinner and afterwards I use dental floss.

After después de, Spanish often uses an infinitive when the subject is the same as in the main clause.

Here, the same person does both actions:

• uso hilo dental
• cenar

So después de cenar means after eating dinner / after having dinner.

This is very common in Spanish:

• antes de salir = before leaving
• después de estudiar = after studying

Yes. Both are possible, but they are slightly different in feel.

• después de cenar = after having dinner / after eating dinner
• después de la cena = after dinner (more focused on the meal as a thing/event)

In everyday speech, después de cenar sounds very natural.

Yes, siempre can move around, but the original order is very natural and neutral.

Common possibilities:

• Siempre uso hilo dental después de cenar.
• Uso siempre hilo dental después de cenar.
• Después de cenar, siempre uso hilo dental.

The first one is probably the most straightforward and natural for a learner to use.

Usar hilo dental is completely correct and very natural.
But another common way is:

• Siempre me paso el hilo dental después de cenar.

This literally means something like I pass the dental floss over my teeth, but in real usage it means I floss.

So both work:

• uso hilo dental = clear, neutral
• me paso el hilo dental = also very natural and a bit more idiomatic

In most of Spain, c before e is pronounced like the th in think.

So cenar sounds roughly like:

• theh-NAR in much of Spain

In other Spanish-speaking places, and also in some parts of Spain, it may sound more like:

• seh-NAR

Also:

• hilo: the h is silent
• después: stress is on the second syllable because of the accent mark: des-PUÉS

It means a routine or habit.

The clues are:

• siempre = always
• uso in the present simple = habitual action

So the sentence is understood as something the speaker regularly does, not just something happening once.

Sometimes hilo dental can also refer to a thong in other contexts, but not here. In this sentence, the meaning is completely clear because of the context:

• usar hilo dental
• después de cenar

That strongly points to dental floss, not clothing. So there is no real ambiguity in normal use.