البن المطحون في الفلتر.

It can be understood as a full sentence in Egyptian Arabic:

• البن المطحون في الفلتر
• The ground coffee is in the filter

Arabic often leaves out the present-tense verb is / are. So there is no separate word for is here.

Depending on context, it could also be read as a phrase meaning:

• the ground coffee in the filter

But if the intended meaning shown to the learner is The ground coffee is in the filter, then this is a normal verbless present-tense sentence.

البن usually means coffee beans or coffee grounds / coffee as a substance.

In Egyptian Arabic:

• بن = coffee beans, ground coffee, coffee powder
• قهوة = coffee as a drink, or coffee in a broader/general sense

So in this sentence, البن is a very natural choice because we are talking about coffee that has been ground and placed in a filter.

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

So:

• البن المطحون = literally the coffee ground
• natural English = the ground coffee

This is one of the big word-order differences between English and Arabic.

Because the adjective has to match the noun in definiteness.

Here:

• البن = the coffee
• المطحون = the ground

Together:

• البن المطحون = the ground coffee

If you make the noun definite, the adjective also becomes definite.

Compare:

• بن مطحون = ground coffee / some ground coffee
• البن المطحون = the ground coffee

المطحون literally means ground or that has been ground.

It comes from the verb:

• طحن = to grind

So مطحون is like a passive-participle-type form meaning:

• ground
• grinded up in the sense of being processed into powder or smaller pieces

It is commonly used with things like:

• coffee
• spices
• grains

A simple learner-friendly pronunciation is:

• el-bunn el-maṭḥoon fil-filter

A few useful notes:

• البن is often pronounced el-bunn
• المطحون is el-maṭḥoon
• في الفلتر often sounds like fil-filter in connected speech

That last part happens because في + الـ often blend together in natural speech:

• في الفلتر → fil-filter

في is the normal everyday word for in or inside in Egyptian Arabic.

So:

• في الفلتر = in the filter

You do not need a separate word here. English sometimes distinguishes in and inside, but Arabic often just uses في for both, depending on context.

It is a very common loanword in Egyptian Arabic, borrowed from English.

So فلتر means filter, and Egyptian speakers use it very naturally in everyday speech.

A more formal or traditional Arabic word might be:

• مصفاة

But in Egyptian colloquial speech, فلتر is extremely normal.

Sometimes yes, but the nuance is a little different.

• البن المطحون strongly suggests ground coffee / coffee grounds
• القهوة المطحونة is understandable, but may sound less precise in this context

If you are talking about the coffee substance that goes into a filter, البن المطحون is very natural.

An indefinite version would be:

• بن مطحون في فلتر

This would mean something like:

• ground coffee is in a filter
• some ground coffee is in a filter

In your original sentence, the ال on the words makes them definite:

• البن = the coffee
• الفلتر = the filter

That is why the original sounds like the ground coffee is in the filter.