"The man has an airplane."
Translation:L'homme a un avion.
Not native, but perhaps I can help.
Cela and cet do not mean "the."
Cet = that
L' = the
Cela can mean "it" or "that." It is not an adjective like cet, but rather a indefinite demonstrative pronoun. It replaces an unspecified or abstract noun such as a situation or an idea.
Nous avons pensé à cela. = We thought of that.