Min veninde rejser til havet i juli, men i august bliver hun hjemme og arbejder.

Veninde means female friend.
Ven means friend and is usually understood as a male friend, or sometimes just friend in older/general usage.

So:

• min ven = my friend / my male friend
• min veninde = my female friend

A native English speaker may notice that Danish often marks this distinction more clearly than English does.

The possessive adjective changes according to the gender and number of the noun.

Veninde is a common gender singular noun, so you use min:

• min veninde = my female friend

Compare:

• min bil = my car
• mit hus = my house
• mine venner = my friends

So the pattern is:

• min
  • common gender singular
• mit
  • neuter singular
• mine
  • plural

Danish very often uses the present tense to talk about the future, especially when there is a clear time expression.

Here, i juli makes the future time obvious, so rejser is completely natural:

• Min veninde rejser til havet i juli = My female friend is going/traveling to the sea in July

This is similar to English sentences like:

• She leaves in July
• We travel next week

So the present tense in Danish does not always mean present time.

Rejser is the present tense of at rejse, which means to travel.

It often suggests going on a trip or journey, rather than simply moving from one place to another. In this sentence, it fits well because the person is going somewhere for a period of time.

Compare:

• at rejse = to travel
• at gå = to walk/go
• at køre = to drive/go by vehicle
• at tage til = to go to

So rejser til havet means something like travels/goes to the seaside.

Yes, til havet literally means to the sea, but in context it often means to the seaside / to the coast.

A learner might expect to the beach, which would usually be:

• til stranden = to the beach

So:

• til havet can sound broader, like going to the sea/coast area
• til stranden is more specifically to the beach

Also notice that havet is the definite form of hav:

• et hav = a sea
• havet = the sea

In Danish, months usually take the preposition i:

• i juli = in July
• i august = in August
• i december = in December

This is the normal pattern for months.

Unlike English, Danish does not use an article here. So you just say:

• i juli, not i den juli or anything similar

This is because Danish is a V2 language. That means the finite verb normally comes in the second position in a main clause.

If you start the sentence with a time expression like i august, that takes the first position, so the verb must come next:

• I august bliver hun hjemme

Structure:

1. I august
2. bliver
3. hun
4. hjemme
5. og arbejder

If you begin with the subject instead, then the verb still stays second:

• Hun bliver hjemme i august

This word order is one of the most important features of Danish grammar.

That is a very common question. Blive often does mean become, but it can also mean remain or stay, depending on the context.

In the expression blive hjemme, it means:

• stay at home

So:

• hun bliver hjemme = she stays home / she stays at home

Compare:

• Hun bliver lærer = She becomes a teacher
• Hun bliver hjemme = She stays home

So the meaning of blive depends on what comes after it.

This is an important distinction:

• hjemme = at home, home as a location
• hjem = homeward, home as a destination/direction

So:

• Hun er hjemme = She is at home
• Hun bliver hjemme = She stays at home
• Hun går hjem = She goes home

In your sentence, hjemme is correct because it describes where she remains.

Because the subject is the same for both verbs, Danish can leave it out in a coordinated structure.

So:

• bliver hun hjemme og arbejder

means:

• she stays home and works

The subject hun applies to both verbs:

• bliver
• arbejder

You could think of it as:

• I august bliver hun hjemme, og hun arbejder

But repeating hun is not necessary here.

Because men joins two main clauses here:

• Min veninde rejser til havet i juli
• men i august bliver hun hjemme og arbejder

A comma before men is normal in Danish in a sentence like this. It helps show the contrast between the two parts:

• in July she travels
• but in August she stays home and works

So the comma is both standard and helpful for clarity.