Saya menggosok gigi seri lalu memakai benang gigi.

Gigi seri means incisors, or the front teeth.

So the sentence literally says:

I brushed my incisors/front teeth, then used dental floss.

That is a bit narrower than English I brushed my teeth. If you mean all your teeth, the more natural Indonesian would usually be just gigi, for example:

Saya menyikat gigi lalu memakai benang gigi.

In Indonesian, possessives are often left out when they are obvious from context.

So menggosok gigi naturally means to brush one’s teeth, and in a sentence with saya, it is understood as my teeth.

You could say gigi saya, but it is often unnecessary unless you want emphasis or contrast.

• Saya menggosok gigi. = I brush/brushed my teeth.
• Dia menggosok giginya. = He/She brushes/brushed his/her teeth.

Yes, gosok basically means rub, scrub, or brush by rubbing. But in Indonesian, menggosok gigi is a common way to say brush your teeth.

That said, many speakers also say:

• menyikat gigi = to brush one’s teeth

This is often a bit more exact, since sikat means brush.

So both can work, but:

• menggosok gigi = common and natural
• menyikat gigi = also natural, and a little more literally tied to using a toothbrush

Lalu means then, after that, or and then.

It shows sequence: first one action happens, then the next one.

In this sentence:

• Saya menggosok gigi seri lalu memakai benang gigi.
• I brushed my front teeth, then flossed.

Other similar words are:

• kemudian = then, afterwards
• terus = then, next, and then (more conversational in some contexts)

Lalu is very common and neutral.

Indonesian often expresses this idea with a phrase rather than a single dedicated everyday verb.

So memakai benang gigi means to use dental floss.

That is a very normal way to express to floss.

You may also see:

• menggunakan benang gigi = to use dental floss
• membersihkan gigi dengan benang gigi = to clean the teeth with dental floss
• membersihkan sela-sela gigi dengan benang gigi = to clean between the teeth with dental floss

So Indonesian usually explains the action instead of using one compact verb like English floss.

Yes. Benang gigi is the standard Indonesian term for dental floss.

Literally it is:

• benang = thread
• gigi = tooth/teeth

So it is basically tooth thread.

In casual speech, some people may also use English-influenced terms, but benang gigi is the standard and safest choice.

Both are verbs with the meN- active verb prefix, which is very common in Indonesian.

• gosok → menggosok
• pakai → memakai

This form is typically used when the subject is actively doing something, especially to an object.

Here:

• Saya menggosok gigi seri = I brush/brushed the front teeth
• (Saya) memakai benang gigi = I use/used dental floss

A useful thing to notice is that the prefix changes shape depending on the first sound of the root:

• meN- + gosok → menggosok
• meN- + pakai → memakai

You do not need to memorize every sound change immediately, but it is good to recognize that these are normal active verb forms.

Both are possible, but they sound different in tone.

• saya = neutral, polite, standard
• aku = more casual, personal, intimate

So:

• Saya menggosok gigi seri lalu memakai benang gigi. sounds neutral or slightly formal.
• Aku menggosok gigi seri lalu memakai benang gigi. sounds more casual.

If you are learning standard Indonesian, saya is a very safe default.

It is understandable, but there is one thing that may sound a little unusual: gigi seri specifically means incisors/front teeth.

So if you really mean only the front teeth, the sentence is fine.

But if the intended meaning is the more general English idea I brushed my teeth and then flossed, a more natural version would usually be:

• Saya menyikat gigi lalu memakai benang gigi.
• Saya menggosok gigi lalu memakai benang gigi.

If you want to be especially clear about flossing between the teeth, you could say:

• Saya menyikat gigi lalu membersihkan sela-sela gigi dengan benang gigi.

So the original sentence is grammatical, but it is a bit more specific than the usual English meaning.

Yes. Both are possible.

• memakai benang gigi = use dental floss
• menggunakan benang gigi = use dental floss

The difference is mostly about style:

• memakai often feels a bit more everyday and flexible
• menggunakan can feel slightly more formal or explicit

In this sentence, either works well.

Yes, absolutely.

That is a very normal kind of translation. Even if the Indonesian wording is literally used dental floss, the natural English translation is often simply:

then I flossed

This is common in translation: you do not have to match each word literally if the meaning is the same and the English becomes more natural.