אני לא רוצה סתימה נוספת, ולכן אני מצחצחת שיניים גם אחרי ארוחת צהריים.

Hebrew present-tense verbs agree with the subject in gender and number.

• אני מצחצחת = I brush / I am brushing said by a female speaker
• A male speaker would say אני מצחצח

The word רוצה in unpointed Hebrew is spelled the same for masculine and feminine, but with vowels it would be:

• masculine: רוֹצֶה
• feminine: רוֹצָה

So in this sentence, מצחצחת is the clearest sign that the speaker is female.

Because in Hebrew, adjectives normally come after the noun they describe.

So:

• סתימה נוספת = an additional filling / another filling

This is the normal Hebrew order:

• noun + adjective

Also, the adjective must agree with the noun in gender and number:

• סתימה is feminine singular
• נוספת is feminine singular

Yes, definitely. Both can mean another filling.

The difference is mostly style and nuance:

• עוד סתימה = more everyday, very common in speech
• סתימה נוספת = a bit more formal or explicit, like an additional filling

In this sentence, סתימה נוספת sounds natural and slightly more polished.

Because the phrase is indefinite: another filling, not the additional filling.

Compare:

• סתימה נוספת = another / an additional filling
• הסתימה הנוספת = the additional filling

Since the speaker is talking about not wanting any new filling, Hebrew uses the indefinite form with no ה־.

ולכן means and therefore, and so, or which is why.

It is made of:

• ו־ = and
• לכן = therefore / so

So:

• אני לא רוצה סתימה נוספת, ולכן... = I don’t want another filling, and therefore...

It connects the first idea to its result.

Because לצחצח שיניים is a very common Hebrew expression, similar to an idiomatic phrase.

Here שיניים is being used in a general sense, so you do not need את.

Compare:

• אני מצחצחת שיניים = I brush my teeth / I do tooth-brushing
• אני מצחצחת את השיניים = I brush the teeth / my teeth
  This is also possible, but it sounds more definite and specific.

So the version in your sentence is very natural and common.

Hebrew often leaves out possessive words like my when the meaning is obvious, especially with body parts and personal routines.

So:

• מצחצחת שיניים naturally means brush my teeth
• You do not need to say את השיניים שלי

If you said את השיניים שלי, it would sound more emphatic, like you are stressing my teeth specifically.

It is a construct phrase.

Literally:

• ארוחה = meal
• צהריים = noon / midday
• ארוחת צהריים = meal of noon = lunch

In Hebrew, this noun relationship is very common. The first noun changes form into the construct state:

• ארוחה → ארוחת

So this is not just two separate nouns side by side; it is a fixed grammatical structure.

Because the sentence means after lunch in a general sense, not after the lunch.

So:

• אחרי ארוחת צהריים = after lunch
• אחרי ארוחת הצהריים = after the lunch or after that specific lunch

English often uses no article with meal words, and Hebrew often does something similar here.

Here גם means also.

Its position shows what it is focusing on. In this sentence:

• גם אחרי ארוחת צהריים = also after lunch

That means the speaker brushes at other times too, and lunch is one more time when she does it.

If you moved גם, the emphasis could shift slightly. Hebrew is flexible, but in this sentence the placement is very natural.

Because Hebrew present tense can express both:

• something happening right now
• something done regularly / habitually

So:

• אני מצחצחת שיניים can mean I am brushing my teeth
• or I brush my teeth

Here, because of the context, it clearly means a habit: she brushes her teeth after lunch as a regular practice.

Yes, they share צהריים, but they mean different things.

• ארוחת צהריים = lunch
• אחר הצהריים = the afternoon

So:

• אחרי ארוחת צהריים = after lunch
• אחר הצהריים = in the afternoon / the afternoon

Learners often mix these up because they look similar, but they are different expressions.