WELCOME BACK TO THE START / BEGINNING / COMMENCEMENT OF A NEW SCHOOL YEAR
What are the differences between these three words that appear to have the same definition of commencer in French.
In a large sense, start and begin have the exact same meaning and thus can be used interchageably like in the sentences below:
It began raining or It began to rain*
It started raining or It started to rain.
However, when we look at the orgins of these two words we could see that in certain situations, it makes more sense to use one verb over the other, though both are still correct in modern, everday speech.
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The verb begin comes from the Old English verb beginnan which literally has the idea of "to attempt" (tenter de faire quelque chose) or "to undertake" (entreprendre).
So it might make more sense to say, to begin a test, since it is something that must be attempted or undertook because of the difficutly it might. However, just to clarify, it's perfectly correct to say, to start a test.
The verb start comes from the Old Englsih verb steortian and the Kentish styrtan. The idea of these word were "to leap up" (se lever d'un bond, sauter, bondir). What I find inetersting is that the root prefix of these words come from the Proto-Indoeuropean "ster-" which means "stiff" (raide, crispé).
So you could see why in a sports race, we cry out "Ready! Set! Start!" to the runners lined up in row ready to run. They are all in a fixed position ready to jump forward on command. To use begin in that expression would just seem unnatural, though we never think about why.
It would thus seem odd to use the verb "begin" in expression that have the meaning of "turning on" such as in to start the car or to start a computer. Try interchanging "begin" in those exressions. It doesn't work!
This idea of "leaping up" can also be found in the expression to give someone a start which mean to startle (surprendre, faire sursauter). So, then the verb to startle has the exact same Old English origin.
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The verb commence came into the English language around the year 1300, unsurprisingly to my French learners of English, from the Old French word comencier. Of course that Old French word came to be them modern French word commencer and finds its origins from the Latin cominitiare which meant "to initiate a priest" or "to consecrate."
Most words in the English language that are derived from the Old French are more formal. For example, in to start a meeting that is more official and formal in nature we might say, "Shall we commence," whereas in a more informal meeting we would more likely say "Let's begin" or "Let's start."
It is lexically correct to say, "It commenced raining," but if you were to say that in everyday speech, you would probably get some strange looks. It's sounds very unnatural.
You can find this idea of "initiation" in the word commencement in Amercian English which refers to the graduation ceremony (remise des diplômes) at the end of high school or university because in a sense, the students are being initiated into a new stage of their life.
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So, now to you!! Why not pick up a dictionary or find one online and look up other expressions derived from or containng begin, start or commence.
*Note: both verbs can be followed by either the infinitive form of the verb (to + verb base form) or the gerund form (verb +-ing).
Have you ever glamped? Do you glamp? No, contrary to how it may sound at first, I’m not trying to be rude. 
