В августе море тёплое, и на пляже много людей.

Because after в when it means in a time period, Russian normally uses the prepositional case.

• август = August
• в августе = in August

This is a very common pattern:

• в январе = in January
• в мае = in May
• в году = in the year

So в августе means in August.

Because море is a neuter noun, and adjectives in Russian must agree with the noun in gender, number, and case.

• море = sea, and it is neuter
• neuter singular adjective form of тёплый is тёплое

So:

• тёплый = masculine
• тёплая = feminine
• тёплое = neuter
• тёплые = plural

That is why море тёплое means the sea is warm.

In Russian, the present-tense verb to be is usually not expressed.

So English The sea is warm becomes simply:

• море тёплое

Russian often leaves out есть in sentences like this. That is completely normal.

Compare:

• Он студент. = He is a student.
• Дом большой. = The house is big.
• Море тёплое. = The sea is warm.

Because на here means on / at, and after на when talking about location, Russian uses the prepositional case.

• пляж = beach
• на пляже = on the beach / at the beach

This is similar to:

• на столе = on the table
• на работе = at work
• на улице = outside / on the street

So на пляже много людей means there are many people on the beach.

Because it changes into the prepositional case after на when describing location.

• dictionary form: пляж
• prepositional singular: пляже

This kind of ending is common for many masculine nouns:

• в музее = in the museum
• в саду = in the garden
• на пляже = on the beach

So the ending changes because the grammar requires the prepositional case.

These two expressions mean different things.

• много людей = many people / a lot of people
• многие люди = many people, but more like many people in general or many people do...

In this sentence, we are talking about quantity present in a place, so много людей is the natural choice.

Also, after много, Russian uses the genitive plural:

• люди is the normal plural of человек
• but after много, you need людей

So:

• люди = people
• много людей = many people

Because много requires the genitive case, specifically the genitive plural here.

The noun человек has:

• singular: человек
• nominative plural: люди
• genitive plural: людей

So:

• люди на пляже = people are on the beach
• много людей на пляже = many people are on the beach

After words like много, мало, несколько, Russian often uses the genitive:

• много книг = many books
• мало времени = little time
• несколько студентов = several students

Russian often expresses there is / there are without a separate word like English there is.

So:

• на пляже много людей literally looks like on the beach many people
• but it means there are many people on the beach

Sometimes Russian does use есть, but in simple present-tense existence statements, it is often omitted.

For example:

• В комнате стол. = There is a table in the room.
• В городе много машин. = There are many cars in the city.
• На пляже много людей. = There are many people on the beach.

Yes, it could be changed, but the original order is the most natural and neutral.

Original:

• В августе море тёплое, и на пляже много людей.

This presents:

1. the time: В августе
2. statement about the sea
3. statement about the beach

Russian word order is more flexible than English, but changing it often changes the focus or emphasis.

For example:

• Море в августе тёплое... puts a bit more focus on the sea
• Много людей на пляже... sounds more focused on the number of people

The given version is smooth and natural for a general statement.

Because и is joining two separate clauses:

1. В августе море тёплое
2. на пляже много людей

Each part has its own predicate structure, so Russian normally uses a comma before и in this kind of compound sentence.

So the comma is correct and expected.

In this sentence, it can be understood as either on the beach or at the beach, depending on the English translation style.

Russian на пляже literally corresponds to on the beach, but in natural English you might also say at the beach.

So:

• на пляже много людей = there are many people on the beach / at the beach

Both are fine translations.

The letter ё is pronounced roughly like yo.

So тёплое is pronounced approximately:

• TYOP-la-ye

The ё shows the stress and the yo sound:

• тёплый
• тёплое

In printed Russian, ё is sometimes written as е, so you may also see теплое, but the correct pronunciation is still тёплое.

For learners, it is useful to remember that ё is not the same as plain е.

You could say something like that, but it is a different phrasing and slightly more literary or descriptive.

The original:

• и на пляже много людей

is simple, neutral, and very natural.

Your alternative:

• пляж полон людей = the beach is full of people

This is also understandable, but it has a stronger, more expressive tone.

So for basic everyday Russian, на пляже много людей is the safer and more common pattern.

It has two linked parts:

1. В августе море тёплое

   • В августе = time expression
   • море = subject
   • тёплое = predicate adjective

2. и на пляже много людей

   • и = and
   • на пляже = location
   • много людей = quantity expression meaning many people

So the sentence is basically:

• In August, the sea is warm, and there are many people on the beach.

This is a very common Russian pattern:

• time/location first
• then the main information
• often with to be omitted in the present tense