The passive form is sometimes used to make a different noun the subject. In the first sentence, the *construction* of the Panama Canal is what is important. To highlight the one(s) who constructed the Panama Canal, we could turn the sentence around: "Ferdinand de Lesseps began to construct the Panama Canal in 1881, but the canal was abandoned and later completed by the US between 1904 and 1914."

Basically, in passive sentences the subject receives the action instead of doing the action. I'll use your first example, as we have both the receiver of the action (the Panama Canal) and the performer (Ferdinand de Lesseps). "The Panama Canal's construction was begun (by Ferdinand de Lesseps) in 1881." We can actually take Ferdy out of the sentence completely. The topic is the Panama Canal. Passive forms are very useful when we don't know who did the action... or don't want to say who did it. (Compare "Your lunch has been eaten" to "I have eaten your lunch"!) So let's look at the active form: "Ferdinand de Lesseps began construction on the Panama Canal in 1881." The topic is M. de Lesseps, and not the Panama Canal. The rest of the paragraph should continue to be about M. de Lesseps, otherwise you have to start a new paragraph. Trivia: the Panama Canal is 82km long. ...far canal!

Began - past tense of be·gin (Verb) Something that has already started or is used as a starting point or place. For example: "the ground campaign had begun". To begin - Something that hasn't started yet and the event has yet to take place. Hope that helps :)