Весь день моросил дождь, поэтому парк был почти пустым.

Весь день means all day.

Here весь means the whole / all of. In this sentence, весь день is a very common way to express duration of time.

• весь день = all day
• всю ночь = all night
• всё утро = all morning

You could also say целый день, which often means the whole day or for the entire day. In many contexts, весь день and целый день are very close in meaning.

A small nuance:

• весь день = neutral, very common
• целый день = can sometimes feel a little more emphatic, like the entire day long

In this sentence, весь день sounds completely natural.

день is in the accusative case, because Russian often uses the accusative to show duration of time.

So:

• весь день = for the whole day / all day
• всю неделю = all week
• весь год = all year

For masculine inanimate nouns like день, the accusative singular looks the same as the nominative singular:

• nominative: день
• accusative: день

That is why you do not see a different ending here.

Моросил comes from моросить, which means to drizzle or to rain lightly in a fine, misty way.

So моросил дождь is literally something like:

• the rain was drizzling
• more naturally in English: it drizzled / a light rain was falling

This verb gives a more specific image than just шёл дождь.

Compare:

• шёл дождь = it was raining
• моросил дождь = it was drizzling

So моросил suggests light, fine, unpleasant rain rather than a strong downpour.

It can feel slightly redundant from an English point of view, but it is normal in Russian.

Russian often uses a weather verb together with дождь:

• шёл дождь = rain was falling / it was raining
• моросил дождь = it was drizzling
• лил дождь = rain was pouring down

So моросил дождь is a natural Russian pattern.

You can also hear impersonal versions in some contexts, but this full version is very standard and clear.

Because it agrees with дождь, which is a masculine singular noun.

In the past tense, Russian verbs usually show gender and number:

• masculine: моросил
• feminine: моросила
• neuter: моросило
• plural: моросили

Since дождь is masculine singular, the verb is моросил.

The comma separates two parts of the sentence:

1. Весь день моросил дождь
2. поэтому парк был почти пустым

Here поэтому means therefore / so / that’s why, and it connects the second clause to the first as a result.

So the comma is natural because the sentence has:

• a cause: it drizzled all day
• a result: therefore, the park was almost empty

In English, this often corresponds to:

• It drizzled all day, so the park was almost empty.

Поэтому means therefore, so, or that’s why. It introduces a result.

• Весь день моросил дождь, поэтому парк был почти пустым. = It drizzled all day, so the park was almost empty.

Потому что means because. It introduces a reason.

Compare:

• Весь день моросил дождь, поэтому парк был почти пустым. = It drizzled all day, so the park was almost empty.

• Парк был почти пустым, потому что весь день моросил дождь. = The park was almost empty because it drizzled all day.

So:

• поэтому = result
• потому что = reason

This is about the predicate adjective after был.

In Russian, when a noun is described after быть in the past or future, the adjective can often appear in the instrumental case:

• парк был пустым

That is why you see пустым: it is the instrumental singular masculine form of пустой.

This construction is very common and natural.

There are also other possibilities in Russian, depending on style and nuance:

• парк был почти пуст — also very natural; this uses the short form adjective
• парк был почти пустой — possible in some contexts, but less standard here than пустым or пуст

So in this sentence, был почти пустым is a very normal choice.

Both can mean empty, but they are slightly different in style and structure.

• парк был почти пустым
  uses the long adjective in the instrumental
• парк был почти пуст
  uses the short adjective

Both are correct and natural.

A rough impression:

• пуст can sound a bit more concise and statement-like
• пустым can feel a bit fuller or more descriptive

In modern Russian, both patterns are common, though some adjectives prefer one pattern more than the other. With пустой, both are very possible.

Почти means almost, and it normally goes right before the word it modifies.

So:

• почти пустым = almost empty

This is similar to English:

• almost empty
• almost impossible
• almost ready

If you moved почти, the emphasis or structure would change, and in this sentence почти пустым is the natural placement.

Yes, the word order can change, but the original is very natural.

Original:

• Весь день моросил дождь, поэтому парк был почти пустым.

This is smooth and neutral:

• time first: Весь день
• then the event: моросил дождь
• then the result: поэтому парк был почти пустым

You could also hear variations like:

• Дождь моросил весь день, поэтому парк был почти пустым.
• Поэтому парк был почти пустым would usually not come first unless the context strongly supports it.

Russian word order is more flexible than English, but the choice affects emphasis. The original sentence sounds very natural for neutral narration.

Yes, absolutely.

• Дождь шёл весь день = it rained all day
• Весь день моросил дождь = it drizzled all day

The difference is mainly vocabulary:

• шёл дождь is more general
• моросил дождь is more specific and means the rain was light and drizzly

So if you want the exact image of fine drizzle, моросил is better.

Because парк is the subject of the second clause:

• парк был почти пустым

The subject of a clause is normally in the nominative case.

So:

• парк = nominative singular
• был agrees with it in masculine singular
• пустым describes it as part of the predicate

It mostly presents a background situation.

The verb моросил is imperfective past, which is very common for describing an ongoing process or repeated/extended action in the past. Since the sentence says весь день, the idea is that the drizzling continued over the course of the day.

So the feeling is:

• for the whole day, rain was drizzling
• and as a result, the park was almost empty

This is why моросил works well: it emphasizes duration, not a single completed moment.

A few useful stress marks:

• весь день
• мороси́л
• дождь
• поэ́тому
• парк
• почти́
• пусты́м

A couple of pronunciation notes:

• дождь can be tricky for English speakers because of the consonant cluster.
• поэтому is commonly pronounced as поэ́тому, though in fast speech it can sound a bit compressed.
• In пустым, the stress is on the second syllable: пусты́м.

It sounds mostly neutral and natural, with a slightly descriptive tone because of моросил.

• Весь день моросил дождь is a bit more vivid than the plain весь день шёл дождь
• поэтому парк был почти пустым is straightforward and neutral

So overall, the sentence sounds like normal written or spoken Russian, especially in narration or description. It is not overly formal, and it is not slangy.