Unter der Fußmatte vor der Tür liegt ein alter Schlüssel.

German often puts something other than the subject first. In this sentence, Unter der Fußmatte vor der Tür is placed first to emphasize the location.

The important word-order rule is that the conjugated verb must still come in second position, so:

• Unter der Fußmatte vor der Tür = first position
• liegt = second position
• ein alter Schlüssel = subject after the verb

This is very normal in German. English can do something similar, but German does it much more often.

Because the sentence begins with a location phrase, the verb must come second. That pushes the subject after the verb.

So the structure is:

• first element: Unter der Fußmatte vor der Tür
• second element: liegt
• then the subject: ein alter Schlüssel

This is called inversion in many teaching explanations. The subject is still ein alter Schlüssel, even though it comes after the verb.

The preposition unter can take either dative or accusative, depending on meaning.

• dative = location, no movement
• accusative = direction or movement toward somewhere

Here the sentence describes where the key is located, so German uses the dative:

• unter der Fußmatte = under the doormat

If there were movement, you would use accusative:

• Ich lege den Schlüssel unter die Fußmatte.
  = I put the key under the doormat.

So:

• wo? (where?) → dative
• wohin? (to where?) → accusative

For the same reason: vor is also a two-way preposition.

• vor der Tür = in front of the door / by the door → location, so dative
• vor die Tür = to in front of the door → movement toward, so accusative

In this sentence, vor der Tür describes where the doormat is, so the dative is used.

Most naturally, it describes die Fußmatte.

So the phrase means:

• Unter der Fußmatte = under the doormat
• vor der Tür = in front of the door

Together: under the doormat in front of the door

In real usage, the whole location phrase tells you where the key is, but grammatically vor der Tür most directly attaches to der Fußmatte.

It is dative singular.

The noun is:

• die Fußmatte = the doormat

In the dative singular, feminine nouns keep der as the article:

• nominative: die Fußmatte
• accusative: die Fußmatte
• dative: der Fußmatte
• genitive: der Fußmatte

So after unter in a location meaning, you get unter der Fußmatte.

Because ein alter Schlüssel is the subject of the sentence, so it is in the nominative case.

The noun is:

• der Schlüssel = the key

As a masculine noun:

• nominative: ein alter Schlüssel
• accusative: einen alten Schlüssel
• dative: einem alten Schlüssel

Since the key is the thing that lies there, it is the subject, so nominative is required.

Because it describes a masculine nominative singular noun after ein.

The noun phrase is:

• ein alter Schlüssel

Here is the pattern:

• ein alter Schlüssel = masculine nominative
• einen alten Schlüssel = masculine accusative
• einem alten Schlüssel = masculine dative

This adjective ending is part of German adjective declension. In this sentence, alter must match Schlüssel in gender, number, and case.

Liegen means to lie or to be lying, and it is often used for objects that are resting somewhere.

So German prefers:

• Der Schlüssel liegt unter der Fußmatte.

rather than simply using sein.

This is similar to how German often distinguishes position more specifically:

• liegen for things lying flat or resting
• stehen for things standing upright
• sitzen for sitting beings, and sometimes objects in fixed positions

Using ist would usually sound less natural here.

The infinitive is liegen.

Conjugation:

• ich liege
• du liegst
• er/sie/es liegt
• wir liegen
• ihr liegt
• sie/Sie liegen

In the sentence, the subject is ein alter Schlüssel, which is third-person singular, so the form is liegt.

Because nouns in German have grammatical gender, and Schlüssel happens to be masculine:

• der Schlüssel

There is not always a logical reason that helps learners; gender often just has to be learned with the noun. That is why it is best to memorize nouns with their articles:

• der Schlüssel
• die Fußmatte
• die Tür

That makes it much easier to form correct case endings later.

It is a compound noun:

• Fuß = foot
• Matte = mat

So Fußmatte literally means foot mat, which is the German word for doormat.

German makes compound nouns very freely, and the last part usually determines the grammatical gender. Since Matte is feminine (die Matte), die Fußmatte is also feminine.

ß is called Eszett or scharfes S.

In modern spelling, ß often appears after a long vowel or diphthong. In Fuß, the vowel is long, so it is written with ß.

You may also see ss in other words when the vowel before it is short.

For learners, the main practical point is just to recognize that:

• Fuß = foot
• Fußmatte = doormat

In Switzerland, people usually write ss instead of ß, so you might also see Fussmatte there.

It is understandable, but liegt is more natural.

Using ist only says that the key exists there. Using liegt tells you how it is positioned, and that is the normal German choice for an object like a key.

So:

• ... ist ein alter Schlüssel = possible, but less idiomatic here
• ... liegt ein alter Schlüssel = better, more natural German

It is mostly neutral, but the word order gives a little prominence to the location.

Starting with Unter der Fußmatte vor der Tür makes the sentence feel like it is answering a question such as:

• Wo liegt ein alter Schlüssel?
• Was ist unter der Fußmatte vor der Tür?

If you started with the subject instead:

• Ein alter Schlüssel liegt unter der Fußmatte vor der Tür.

that would also be correct, but it puts the key itself more in focus.

Yes. Both unter and vor are two-way prepositions in German.

That means they can take:

• dative for location
• accusative for movement/direction

In this sentence both are used with dative because everything is about location, not motion:

• unter der Fußmatte
• vor der Tür

This is one of the most important grammar patterns in German.