רופא השיניים אמר שהסתימה הישנה לא טובה, וצריך לעשות סתימה חדשה.

Hebrew often expresses professions like this with a noun chain:

• רופא = doctor
• שיניים = teeth

So רופא שיניים literally means tooth doctor / teeth doctor, i.e. dentist.

When the whole phrase is definite, Hebrew usually puts ה־ on the second noun here:

• רופא שיניים = a dentist
• רופא השיניים = the dentist

This is a very common pattern in Hebrew.

שהסתימה is made of two parts:

• ש־ = that
• הסתימה = the filling

So:

• אמר שהסתימה הישנה לא טובה = said that the old filling isn’t good

In modern Hebrew, ש־ is very often attached directly to the next word.

In Hebrew, adjectives normally come after the noun.

So:

• סתימה ישנה = an old filling
• הסתימה הישנה = the old filling

Also, if the noun is definite, the adjective must be definite too:

• הסתימה = the filling
• הישנה = the old

This matching of definiteness is an important Hebrew rule.

Because סתימה is a feminine singular noun, and adjectives must agree with the noun in gender and number.

So:

• סתימה = feminine singular
• טובה = feminine singular form of good

Compare:

• ספר טוב = a good book
• סתימה טובה = a good filling

That is why the sentence says לא טובה.

Yes. סתימה can mean blockage in some contexts, because the root is connected with closing/stopping up.

But in dental Hebrew, סתימה is the normal word for a filling.

So the meaning depends on context:

• in plumbing: a clog/blockage
• at the dentist: a filling

Here צריך is being used in an impersonal way, meaning something like:

• it is necessary
• one needs to
• we need to

So:

• וצריך לעשות סתימה חדשה = and it’s necessary to do/make a new filling

Hebrew often leaves the subject unstated in this kind of sentence when it is general or obvious from context.

Because צריך here is not describing סתימה.

It is part of the impersonal expression צריך לעשות... = it is necessary to do...

So it does not agree with סתימה חדשה. The new filling is the object of לעשות, not the thing being described by צריך.

In more formal Hebrew, people may also say:

• יש צורך לעשות סתימה חדשה = there is a need to do a new filling

But צריך לעשות... is very common and natural.

That is just the normal Hebrew expression. Hebrew often uses very general verbs like לעשות (to do / make) where English uses a more specific verb.

So:

• לעשות סתימה = to do a filling / to put in a filling

Even if it sounds slightly different from English, it is idiomatic Hebrew.

For the same reason as הישנה earlier: adjectives usually come after the noun in Hebrew.

So:

• סתימה חדשה = a new filling

And because סתימה is feminine singular, the adjective is also feminine singular:

• חדש = new (masculine singular)
• חדשה = new (feminine singular)

Because they mean different things in the sentence:

• הסתימה הישנה = the old filling
  A specific filling that is already in the tooth.

• סתימה חדשה = a new filling
  A new one that needs to be made; it has not been introduced as a specific known item yet.

This definite/indefinite contrast is very natural in both Hebrew and English.

Yes. לא טובה literally means not good.

Depending on context, English may translate it as:

• is not good
• is no good
• isn’t in good condition
• is bad

At the dentist, it usually means the old filling is no longer acceptable or has failed in some way.

Literally it is and it is necessary to. But in natural English, the idea is often smoother as:

• and a new filling needs to be done
• so a new filling is needed
• and you need to get a new filling

So the Hebrew uses ו־ = and, but English may translate the logic more naturally as so in this context.

A common pronunciation would be:

rofé ha-shináyim amár sheha-stimá ha-yeshaná lo tová, ve-tsaríkh la’asót stimá chadashá

A few useful stress points:

• רופֵא → roFÉ
• שיניים → shiNÁyim
• אָמַר → aMÁR
• סתימה → stiMÁ
• ישנה → ye-shaNÁ
• טובה → toVÁ
• צריך → tsaRÍKH
• לעשות → la-aSÓT
• חדשה → cha-daSHÁ

If you want, I can also break the whole sentence down word by word in a learner-friendly table.