המגנט ליד החיישן.

Because Hebrew usually leaves out the verb to be in the present tense.

So המגנט ליד החיישן literally looks like the magnet next to the sensor, but it means:

The magnet is next to the sensor.

In past or future, Hebrew does use a form of to be, for example:

• המגנט היה ליד החיישן = The magnet was next to the sensor
• המגנט יהיה ליד החיישן = The magnet will be next to the sensor

ה־ is the Hebrew definite article, meaning the.

So:

• מגנט = a magnet / magnet
• המגנט = the magnet
• חיישן = a sensor / sensor
• החיישן = the sensor

Unlike English, Hebrew attaches the directly to the beginning of the noun.

ליד means next to, beside, or sometimes near.

So:

• ליד החיישן = next to the sensor

It is a preposition, not a noun or adjective.

No. Even though ליד starts with the letter ל, the whole word ליד is a single preposition meaning next to / beside.

So you should learn ליד as one unit.

Compare:

• לחיישן = to the sensor / for the sensor
• ליד החיישן = next to the sensor

Those are different structures.

The structure is:

subject + location

• המגנט = the subject, the magnet
• ליד החיישן = where it is, next to the sensor

So Hebrew is doing something very similar to English, except that the present-tense is is omitted.

It can be understood as a full sentence:

The magnet is next to the sensor.

But in some contexts, the same words could also be read like a phrase:

the magnet next to the sensor

Usually punctuation, context, and intonation make the meaning clear.

If you want to make the phrase meaning clearer, Hebrew often uses ש־:

• המגנט שליד החיישן = the magnet that is next to the sensor

A simple pronunciation guide is:

ha-mag-NET le-YAD ha-khay-SHAN

A few notes:

• המגנט = ha-magnet
• ליד = leyad
• החיישן = ha-khay-shan

The letter ח in חיישן is pronounced like the ch in Scottish loch or German Bach, not like English h.

Not because of ליד.

ליד is a preposition, so it does not change form to match the noun. You simply put the noun after it:

• ליד החיישן = next to the sensor
• ליד המכשיר = next to the device
• ליד המכוניות = next to the cars

There is no adjective agreement here. That would happen with adjectives, not with ליד.

Hebrew has no separate word for a/an. To make a noun indefinite, you usually just remove ה־.

So:

• מגנט ליד החיישן = A magnet is next to the sensor

And:

• המגנט ליד חיישן = The magnet is next to a sensor

So the presence or absence of ה־ is very important.

Yes, both are normally masculine singular nouns.

That matters if you later add adjectives or use past/future verb forms that show agreement.

For example:

• המגנט קטן = The magnet is small
  masculine singular adjective: קטן
• החיישן חדש = The sensor is new
  masculine singular adjective: חדש

In your sentence, though, you do not see much agreement because the main relation is expressed with the preposition ליד.