Je dors; Nous dormons? But Why?

You will probably be taught that "dormir" is an irregular verb. It certainly seems to be in the present tense but it is not actually. "Dormir" comes from the Latin verb "dormire". The conjugation of this verb in the present tense in Latin was:

Dormio                            Dormimus
Dormis                             Dormitis
Dormit                             Dormiunt

Notice how each conjugation has an 'm'. So, what happened between the time Nero fiddled and the Eiffel tower was built?

If you have been studying French for a while, you will have notice how many times the letters at the ends of words are not pronounced. There is a similar process in certain dialects of English. Thus, for example, some English speakers would pronounce the word "got" as if it did not have a "t". In other words, they might say something like "I go' plans this weekend".

What happened in French over the centuries is that a lot of sounds at the ends of words stopped being pronounced. What you need to understand too is that some sounds in language are weaker than others, that is they are more prone to being dropped. These weaker sounds include 'm' and 'l'.

Let us get back to the French verb, "dormir". Notice that the stem is "dorm", which ends in 'm', a weak sound. Theoretically, the conjugations for this verb in the singular of the present should be:

Je dorm
Tu dorm
Il dorm
Elle dorm
[NOTE: I have deliberately misspelt these words to avoid unnecessary complications.]

Notice how each of these words is pronounced the same way. As time passed, French speakers stopped pronouncing the 'm', leading to the current pronunciation of:

Je dor
Tu dor
Il dor
Elle dor
[NOTE: Again, I have deliberately misspelt the conjugations.]

What about the plural? Since the 'm' is followed by 'ons' and 'ez' for "nous" and "vous" respectively, it is no longer at the end of the word and can thus not be dropped. This is why we today have:

Nous dormons
Vous dormez

The only mystery that remains is the one for "ils" and "elles". Here 'm' is followed by 'ent'. Of course, 'ent' is silent. That said, it was once pronounced so that 'm' was at one time not the last sound in the verb.

For all the reasons listed above, plus one that I did not mention, the conjugation of "dormir" in the present tense in modern French is:

Je dors                         Nous dormons
Tu dors                        Vous dormez
Il dort                           Ils dorment
Elle dort                       Elles dorment

Most verbs in French, if not all, are regular. They only seem irregular if you do not know the history behind them and all the other good things that happen in languages, like sound changes. If you want, you can read up on Phonetics, Phonology and Morphology.