Learn French: Mon camion est celui à côté du garage. - My truck is the one next to the garage.

In French, you can't translate "the one" literally as "le un". You must use a demonstrative pronoun to avoid repeating the noun. Since it replaces le camion (a masculine, singular noun), we use celui. If it were a feminine noun like la voiture, you would use celle.

The lesson teaches celui-ci and celui-là. Why doesn't celui have a suffix in this sentence?

You drop the -ci (this) and -là (that) suffixes when the pronoun is immediately followed by a description, like the location phrase à côté du garage. You generally only use the suffixes when pointing out "this one" or "that one" without adding extra descriptive details after it.

Why is it du garage instead of de le garage?

The expression for "next to" is à côté de. Because the word garage is masculine, it takes the article le. In French, the words de and le must always combine (contract) to become du.

Could I say Mon camion, c'est celui... instead?

Yes! While the sentence shown is perfectly correct, spoken French often uses c'est for emphasis. Saying Mon camion, c'est celui à côté du garage (My truck, it's the one next to the garage) sounds very natural and is common in everyday conversation.