يتمضمض صديقي بالماء.

• يتمضمض = he rinses his mouth / he is rinsing his mouth
• صديقي = my friend
• بالماء = with water

So the sentence structure is:

verb + subject + prepositional phrase

That is a very normal Arabic sentence pattern.

Because Arabic very commonly uses verb-first word order, especially in straightforward statements.

So:

• يتمضمض صديقي بالماء = a normal verb-subject sentence
• صديقي يتمضمض بالماء is also possible, but it puts more attention on my friend

Both are grammatical, but the first one is very natural in standard Arabic prose.

يتمضمض is an imperfect verb, third-person masculine singular.

That means it matches he.

The beginning يـ is the usual imperfect marker for he in many verbs.

Its dictionary form is تمضمض meaning to rinse one’s mouth.

So يتمضمض means:

• he rinses his mouth
• he is rinsing his mouth

depending on context.

It can mean either one.

The Arabic imperfect often covers both:

• a present ongoing meaning: is rinsing
• a habitual/general meaning: rinses

So without extra context, this sentence could mean either:

• My friend is rinsing his mouth with water
• My friend rinses his mouth with water

Context tells you which is intended.

Because it is made of:

• صديق = friend
• ـي = my

So:

• صديق = friend
• صديقي = my friend

This ـي is a attached possessive pronoun.

Because once a noun has a possessive ending like ـي meaning my, it is already definite.

So Arabic says:

• صديقي = my friend

not:

• الصديقي

In other words, possession already makes the noun definite, so ال is not added.

Because this verb normally uses the preposition بـ to show the substance used for rinsing.

So بالماء means with water.

If you removed بـ, the sentence would no longer have the normal standard structure for this verb.

Also, after بـ, the noun is in the genitive case, so in full vocalization it is:

بالماءِ

In Arabic, mass nouns and substances are often used with ال in places where English would simply say water.

So بالماء may still be translated naturally as:

• with water

even though it literally contains ال.

In context, بالماء can sound perfectly normal and does not always need a special emphasis like with the water in English.

Because the meaning is already built into the verb تمضمض.

This verb specifically means to rinse the mouth, so Arabic does not need to add mouth again unless there is a special reason to emphasize it.

That is why يتمضمض بالماء already sounds complete.

A careful full reading would be:

يَتَمَضْمَضُ صَديقِي بِالماءِ

Transliteration:

yatamaḍmaḍu ṣadīqī bil-māʾi

A few useful notes:

• يتمضمضُ ends with ـُ in the normal indicative form
• بالماءِ ends with ـِ because it comes after the preposition بـ
• ماء contains a hamza at the end, so māʾ has a little glottal stop sound

In normal everyday reading, many final case vowels are not pronounced, but it is still useful to know them.