Il reggisella sostiene la sella.

They are the definite articles: il = masculine singular the, and la = feminine singular the.

So in this sentence:

• il reggisella = the seatpost
• la sella = the saddle

Italian articles have to agree with the noun’s gender and number.

Because it is a compound noun. Italian often combines elements into a single word for tools, parts, or devices.

Reggisella is built from:

• reggi- = from reggere, to hold up / support
• sella = saddle

So the word literally suggests something like saddle-support or saddle-holder.

Because noun endings in Italian are helpful patterns, not absolute rules.

A beginner often learns:

• -o → often masculine
• -a → often feminine

But reggisella is a compound noun, and compound nouns do not always follow the usual ending pattern. Its gender is simply learned as part of the word:

• il reggisella

So the article tells you clearly that it is masculine.

Sostiene is the 3rd person singular present tense of sostenere.

Here is the relevant idea:

• sostenere = to support
• il reggisella sostiene = the seatpost supports

It matches the subject il reggisella, which is singular.

A few present-tense forms are:

• io sostengo = I support
• tu sostieni = you support
• lui/lei sostiene = he/she/it supports

So sostiene is the correct form for it supports.

Because sella is the normal word for a saddle on a bicycle, motorcycle, or horse.

By contrast:

• sedile usually means a more general seat, like in a car or chair
• sella is the specific word for a saddle-shaped riding seat

In everyday Italian, especially for bicycles, you may also hear sellino, which is a common informal word for a bike seat.

In a normal full sentence, Italian usually keeps the article with common nouns.

So:

• Il reggisella sostiene la sella. = normal, natural sentence

Leaving the articles out would sound unnatural in ordinary Italian:

• Reggisella sostiene sella = not normal standard sentence Italian

You might see article-less forms in:

• labels
• technical diagrams
• headings
• notes

But in a regular statement, the articles are expected.

The sentence uses the most neutral and common order:

• subject + verb + object
• Il reggisella
  • sostiene
    • la sella

Italian word order is somewhat flexible, but this version is the most straightforward.

You can change the structure for emphasis, but then the sentence may sound more marked or less basic. For example:

• La sella è sostenuta dal reggisella. = The saddle is supported by the seatpost.

That is grammatical, but it is a different structure and less direct than the original.

A rough English-friendly approximation is:

eel rehj-jee-SEL-la sos-TYE-ne la SEL-la

A few useful points:

• reggi- has a j sound, like jam
• sella has a clear double l
• the stress is on -sel- in reggisella
• sostiene is stressed on -tie-

This is only an approximation, but it is a good starting point.

No. Sostenere can also be used in more abstract ways.

It can mean things like:

• to support someone or something
• to maintain / argue / claim an opinion
• to back an idea

But in this sentence it has the very concrete, physical meaning: to hold up / support.

Yes. The second noun is actually built into the first one.

• sella = the saddle
• reggisella = the part that supports the saddle

So the sentence is very transparent once you notice the vocabulary pattern. Italian often forms technical nouns this way, especially for objects and parts. This can be very helpful for learners, because once you know the pieces, the compound word becomes easier to understand and remember.