Webkamera musí být nad monitorem.

to be

Breakdown of Webkamera musí být nad monitorem.

• muset: must/to have to
• nad: above
• monitor: the computer monitor

Questions & Answers about Webkamera musí být nad monitorem.

Why is webkamera in the nominative form?

Because webkamera is the subject of the sentence — it is the thing that must be somewhere.

In Czech, the subject is normally in the nominative case.
The basic dictionary form is webkamera, and that is the form used here.

Also, webkamera is a feminine noun, which you can often guess from the ending -a.

Why is it monitorem and not monitor?

Because the preposition nad requires the instrumental case when it means location, as in above.

So:

• monitor = nominative
• monitorem = instrumental singular

In this sentence, nad monitorem means above the monitor, so Czech uses the instrumental after nad.

Does nad always take the instrumental case?

Not always.

With nad, the case depends on whether you are talking about:

• location → usually instrumental
• movement toward a position → usually accusative

Examples:

• Webkamera je nad monitorem. = The webcam is above the monitor.
  → location, so instrumental
• Pověsil kameru nad monitor. = He hung the camera above the monitor.
  → movement/direction, so accusative

So in your sentence, it is instrumental because it describes where the webcam should be.