Das Gerät ist mangelhaft.

Here das is the definite article, so it means the.

So das Gerät = the device or the appliance.

It is not the pronoun that in this sentence. You can tell because it is directly attached to the noun Gerät.

Because Gerät is a neuter noun in German.

German nouns have grammatical gender:

• der for masculine
• die for feminine
• das for neuter

So the correct dictionary form is:

• das Gerät = the device

This is something you usually just have to learn together with the noun.

Gerät is a fairly general word meaning device, appliance, or sometimes piece of equipment.

Depending on context, it could refer to things like:

• a household appliance
• an electronic device
• a technical instrument
• a machine-like tool

It is broader than one single English word, so the best translation depends on context.

It is pronounced roughly like guh-RAIT, with the stress on the second syllable.

A more accurate guide:

• Ge- sounds like a soft guh
• -rät has a long ä sound, somewhat like air in some accents, but cleaner and tenser

The stress is important: ge-RÄT, not GE-rät.

Mangelhaft is an adjective.

In this sentence it comes after ist, so it is used predicatively:

• Das Gerät ist mangelhaft.

When German adjectives are used this way, they do not take adjective endings.

Compare:

• ein mangelhaftes Gerät = a defective device
  → adjective before the noun, so it gets an ending
• Das Gerät ist mangelhaft = The device is defective
  → adjective after ist, so no ending

Mangelhaft means something like:

• defective
• faulty
• flawed
• not up to standard

It often suggests that there is a defect or shortcoming.

It is a bit more formal than everyday words like kaputt. It can sound especially natural in:

• technical descriptions
• product complaints
• quality control
• legal or commercial contexts

So Das Gerät ist mangelhaft sounds like a formal way to say the device has a defect.

Because it comes after the verb ist.

In German:

• adjectives before a noun usually take endings
• adjectives after verbs like sein, werden, and bleiben usually do not

So:

• das mangelhafte Gerät = the defective device
• Das Gerät ist mangelhaft = The device is defective

This is very common and important in German grammar.

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

Ask: who or what is defective?
Answer: das Gerät

So:

• Das Gerät = subject, nominative
• ist mangelhaft = predicate

Because German main clauses usually follow the verb-second rule.

In a normal statement, the finite verb comes in the second position:

• Das Gerät ist mangelhaft.

Here the first position is taken by Das Gerät, and the second position is ist.

This is one of the most basic German sentence patterns:

• Subject + verb + rest

Yes, you could, but the meaning and tone are a little different.

• Das Gerät ist kaputt. = The device is broken.
• Das Gerät ist mangelhaft. = The device is defective / faulty.

Difference:

• kaputt is common, everyday German
• mangelhaft is more formal and often used when something has a defect, may not meet standards, or is the subject of a complaint

A device can be mangelhaft without being completely unusable.
So mangelhaft is often broader than kaputt.

No. It can also describe other things that are inadequate or substandard.

For example:

• Die Qualität ist mangelhaft. = The quality is poor / defective.
• Die Ausführung war mangelhaft. = The execution was inadequate.

It can even be used in school grading in some contexts, where mangelhaft means something like poor or unsatisfactory.

But in Das Gerät ist mangelhaft, the physical-product meaning is the natural one.

The plural of Gerät is Geräte.

So:

• Das Gerät ist mangelhaft. = The device is defective.
• Die Geräte sind mangelhaft. = The devices are defective.

Notice the changes:

• das → die
• ist → sind
• Gerät → Geräte

You usually negate it with nicht:

• Das Gerät ist nicht mangelhaft. = The device is not defective.

The basic pattern is:

• subject + sein
  • nicht
    • adjective

So nicht comes before the adjective mangelhaft here.