Mein Schlüssel ist weg.

In German, all nouns are capitalized, not just proper names.

So:

• mein Schlüssel = my key
• das Haus = the house
• ein Auto = a car

That is why Schlüssel begins with a capital S.

Because Schlüssel here is masculine singular and it is the subject of the sentence.

The noun der Schlüssel is masculine, so in the nominative singular you use:

• mein Schlüssel = my key

Compare:

• mein Schlüssel = my key
• meine Tasche = my bag
• mein Auto = my car
• meine Schlüssel = my keys

So meine would be wrong here if you mean one key.

It is in the nominative case because it is the subject of the sentence.

In Mein Schlüssel ist weg, the thing doing the being is mein Schlüssel. With the verb sein (to be), the subject is in the nominative.

So:

• mein Schlüssel = nominative subject
• ist = is
• weg = gone / missing / away

Ist is the 3rd person singular form of sein (to be).

The subject is mein Schlüssel, which is singular, so the verb must also be singular:

• ich bin
• du bist
• er/sie/es ist

So:

• Mein Schlüssel ist weg.

If the subject were plural, the verb would change:

• Meine Schlüssel sind weg.

Here, weg means something like gone, missing, or not here anymore.

It often has a slightly everyday, natural feel. It does not necessarily mean the key has been permanently lost; it can simply mean you cannot find it right now.

So weg can suggest:

• it is no longer where it should be
• it has disappeared from view
• you do not know where it is

That is why this sentence is a very common way to say that your key is gone or missing.

Yes, you can, but it feels a bit different.

• Mein Schlüssel ist weg. = My key is gone / missing.
• Mein Schlüssel ist verloren. = My key is lost.

Weg is very common in everyday speech and often sounds more natural in casual situations.

Verloren focuses more directly on the idea of being lost. It can sound a little more formal or more final, depending on context.

In normal conversation, many people would naturally say:

• Mein Schlüssel ist weg.

Because mein already functions as a determiner.

In German, you usually do not say both a possessive and a normal article together before the same noun.

So you say:

• mein Schlüssel = my key

not:

• der mein Schlüssel

This works like:

• der Schlüssel = the key
• ein Schlüssel = a key
• mein Schlüssel = my key

Each of these fills the same slot before the noun.

Here it is singular.

You can tell from two things:

• the possessive form mein
• the verb ist

So:

• Mein Schlüssel ist weg. = singular
• Meine Schlüssel sind weg. = plural

A useful thing to know is that Schlüssel is one of those German nouns whose singular and plural look the same in spelling:

• der Schlüssel = the key
• die Schlüssel = the keys

So the article or determiner usually tells you whether it is singular or plural.

A rough guide is: SHLÜS-sel

Break it up like this:

• Sch sounds like English sh
• lü contains ü, which does not exist in standard English
• ssel sounds a bit like suhl or zel, depending on accent, but with a short vowel before it

The hardest part for English speakers is usually ü.

A useful approximation:

1. Say ee
2. Keep your tongue in that position
3. Round your lips as if saying oo

That gives you something close to German ü.

So Schlüssel is roughly:

• shlüssel

In this sentence, learners often think of weg as acting like an adjective-like predicate after sein.

You do not inflect it here:

• Der Schlüssel ist weg.
• Die Tasche ist weg.
• Die Schlüssel sind weg.

It stays weg.

For a learner, the safest practical point is:

• after sein, weg often behaves like gone in English
• you can learn weg sein as a common expression meaning to be gone / missing

Yes. In a normal statement, German often puts the verb in second position.

The basic version is:

• Mein Schlüssel ist weg.

But you can move another element to the front for emphasis, and the verb still stays in second position:

• Weg ist mein Schlüssel. — possible, but marked and less neutral
• Leider ist mein Schlüssel weg. — Unfortunately, my key is gone.

The neutral, standard word order is:

• Mein Schlüssel ist weg.

So that is the best version to learn first.