بالمجلى في كاسة، وعلى الرف في علبة لبن.

In this kind of Levantine sentence, في is not just the preposition in. It can also mean there is / there are.

So:

• بالمجلى في كاسة = In the sink, there is a glass/cup
• وعلى الرف في علبة لبن = And on the shelf, there is a container of laban/yogurt

A very common Levantine pattern is:

location + في + thing
= in/on/at the location, there is a thing

So here, the first word gives the location, and في introduces what exists there.

Because it is made of three parts:

• بـ = in / at
• الـ = the
• مجلى = sink

Together they become بالمجلى = in the sink or sometimes at the sink, depending on context.

This is very normal in Arabic spelling. Short prepositions like بـ attach directly to the following word.

In present-tense Arabic, sentences often do not need a separate verb like is or are.

In English, you say:

• There is a glass in the sink

In Levantine, you can say:

• بالمجلى في كاسة

Literally, it is more like:

• In-the-sink there-is a-glass

So the idea of there is is expressed by في, not by a verb like English is.

Because the sentence is introducing new objects: a glass and a container.

In Arabic, after existential في meaning there is, the noun is usually indefinite when you mean a/an:

• في كاسة = there is a glass/cup
• في علبة لبن = there is a container of laban/yogurt

If you made them definite, the meaning would change and would sound more like referring to a specific already-known object.

In Levantine, كاسة is a very common word for a drinking vessel. Depending on context, it can mean:

• glass
• cup
• sometimes even mug, loosely

In many everyday situations, English speakers will often translate it as glass. But the exact choice depends on what kind of container is meant.

In Levantine, لبن usually does not mean plain milk. It usually refers to laban, often meaning:

• yogurt
• drinkable yogurt
• a cultured dairy product

If you want milk, the usual word is حليب.

So علبة لبن would usually mean something like:

• a container of yogurt
• a tub/carton of laban

The exact English translation depends on the local food context.

Because the locations are understood differently.

• على الرف = on the shelf
  You use على because something is sitting on top of the shelf.

• بالمجلى = in the sink or sometimes at the sink
  Here بـ is idiomatic and very natural in Levantine for this kind of location.

You may also hear في المجلى in some contexts, but بالمجلى is a very normal colloquial way to say it.

Yes. It is a noun-plus-noun structure often called an idaafa or construct phrase.

It works like this:

• علبة = container / box / tub
• لبن = laban / yogurt

Together:

• علبة لبن = a container of laban

Arabic often uses this structure where English uses of. So instead of saying a container of yogurt with a separate word for of, Arabic simply puts the two nouns together.

Because this is a very natural way to describe what is where.

Starting with the location helps organize the information:

• بالمجلى... = talking about the sink first
• وعلى الرف... = then talking about the shelf

This is especially common when listing items in different places.

English can do something similar:

• In the sink, there’s a glass, and on the shelf, there’s a container of yogurt.

So the word order is natural and helps focus on location first.

It is written الرف, but in pronunciation the ل of الـ assimilates to the ر because ر is a sun letter.

So الرف is pronounced roughly like:

• ar-raff

And على الرف sounds roughly like:

• ʿal ar-raff

The spelling stays الرف, but the pronunciation changes.

Yes, that is also possible in colloquial Arabic, and learners may hear both patterns.

For example:

• في المجلى كاسة
• بالمجلى في كاسة

Both can express There is a glass in the sink, but the feel is a little different.

The version in your sentence puts stronger attention on the location first:

• As for the sink, there’s a glass there
• As for the shelf, there’s a container of laban there

So the sentence you have is especially natural in descriptions or when pointing things out.