Le châssis est sous le véhicule.

Because châssis is a masculine noun in French, so it takes le in the singular.

• le châssis = the chassis
• un châssis = a chassis

French noun gender is something you usually have to learn with the word itself.

Châssis is pronounced roughly like sha-see.

A simple guide:

• ch sounds like sh
• â sounds like a
• ssis sounds like see

The final s is not pronounced.

A common IPA pronunciation is /ʃa.si/.

The accent in â is a circumflex.

In châssis, it is mainly part of the standard spelling. For many modern speakers, it does not make a big difference in pronunciation compared with a plain a.

So for a learner, the important thing is:

• write it as châssis
• pronounce it approximately sha-see

Est is the third-person singular form of the verb être (to be) in the present tense.

Since le châssis is singular, the verb must also be singular:

• Le châssis est... = The chassis is...

If the subject were plural, the verb would change:

• Les châssis sont... = The chassis/chassis are...

Because sous simply means under, and it is followed by the normal article for the noun:

• sous le véhicule = under the vehicle

Du usually means of the or some, so it would not fit here.

Compare:

• sous le véhicule = under the vehicle
• au-dessous du véhicule = below the vehicle

With sous, you normally say sous + article + noun.

Véhicule is pronounced approximately vay-ee-kewl.

Helpful points:

• vé sounds like vay
• the h is silent
• i is pronounced separately, so you hear three syllables
• u is the French u sound, which may be hard for English speakers
• the final e is not pronounced

A common IPA pronunciation is /ve.i.kyl/.

No. In sous le véhicule, you do not pronounce a liaison between sous and le.

So it sounds like:

• sous = soo
• le = luh
• véhicule = vay-ee-kewl

You would say the words separately, without linking a z sound there.

It is singular here.

You can tell because:

• the article is le not les
• the verb is est not sont

So:

• Le châssis est sous le véhicule. = singular
• Les châssis sont sous le véhicule. = plural

A tricky point: châssis is one of those French nouns whose singular and plural are spelled the same. The article and verb help you tell which one it is.

Yes, that is possible, but sous is shorter and more direct.

• sous le véhicule = under the vehicle
• en dessous du véhicule = underneath/below the vehicle

In many cases, sous is the most natural choice when you simply want to state location.

Because this sentence has a very straightforward structure in French:

• Le châssis = subject
• est = verb
• sous le véhicule = prepositional phrase showing location

So it follows the same basic pattern as English:

• The chassis
  • is
    • under the vehicle

This is one reason the sentence is relatively easy for English speakers to understand grammatically.