Der Club öffnet um zehn.

Because Club is a masculine noun in German, so its nominative singular article is der.

• der Club = the club
• ein Club = a club

Grammatical gender in German does not always match logic, so it usually has to be learned together with the noun:

• der Club
• die Tür
• das Haus

In this sentence, der Club is the subject, so the nominative form der is used.

öffnet comes from the verb öffnen, which means to open.

Here it is the 3rd person singular form, because the subject is der Club:

• ich öffne = I open
• du öffnest = you open
• er/sie/es öffnet = he/she/it opens

So Der Club öffnet literally means The club opens.

In everyday English, this often means the club opens for business or the club opens its doors, not that someone is physically opening a door.

Because the subject is der Club, which is 3rd person singular. In the present tense, German verbs change depending on the subject.

For öffnen:

• ich öffne
• du öffnest
• er/sie/es öffnet
• wir öffnen
• ihr öffnet
• sie/Sie öffnen

Since der Club is treated like er grammatically, the correct form is öffnet.

German main clauses usually follow the verb-second rule. That means the conjugated verb comes in the second position.

Here:

1. Der Club = first position
2. öffnet = second position
3. um zehn = the time expression

So the basic structure is:

• Subject + verb + time

This is very normal in German.

You could also move the time expression to the front:

• Um zehn öffnet der Club.

That still works because the verb öffnet remains in second position.

um is the preposition used for specific clock times.

• um zehn = at ten
• um acht Uhr = at eight o’clock
• um halb sieben = at half past six

So in this sentence, um zehn means at ten.

A useful rule:

• um
  • time on the clock
• am
  • day/date
• im
  • month/season/year in some contexts

Examples:

• um zehn = at ten
• am Montag = on Monday
• im Sommer = in summer

It can. Both are possible:

• um zehn
• um zehn Uhr

In everyday German, Uhr is often omitted when the meaning is already clear. So um zehn naturally means at ten o’clock.

Adding Uhr can make it a little more explicit, but it is not necessary here.

By itself, um zehn just means at ten o’clock. It does not tell you whether that is morning or evening.

German often leaves that to context, just like English sometimes does.

If you want to be more specific, you could say:

• um zehn Uhr morgens = at 10 a.m.
• um zehn Uhr abends = at 10 p.m.

With Club, many learners might guess evening, but grammatically the sentence itself does not specify.

The tricky part is ö.

A helpful approximation:

• ö is somewhat like the vowel in British English bird or her, but with rounded lips.

So öffnet is roughly pronounced like:

• URFF-net or ERFF-net, depending on your accent approximation

More exactly:

• ö
• then ff
• then net

Also note:

• the o in Club is usually pronounced similarly to the borrowed English-style word, though often with a German accent
• the ö in öffnet is a completely different vowel from normal o

Yes. Club is used in German and is a borrowed word. It can refer to things like:

• a nightclub
• a social club
• an organization or association

Depending on context, German may also use Klub as a spelling variant, but Club is very common.

So Der Club öffnet um zehn is perfectly normal German.

It is in the nominative case, because it is the subject of the sentence—the thing doing the action.

Ask the question:

• Who opens? → Der Club

That makes der Club nominative.

If the noun were in a different role, the article might change. For example:

• Ich sehe den Club. = I see the club.
  Here den Club is accusative.

Yes, in many situations the German present tense can correspond to either:

• The club opens at ten
• The club is opening at ten

German present tense often covers both a simple present and a present progressive meaning that English separates.

Which English version sounds best depends on context, but the German sentence itself is completely natural as a simple statement of schedule or opening time.