Breakdown of Il m'a dit que sa belle-sœur venait aujourd'hui.
il
he
avoir
to have
son
his/her/its
venir
to come
Questions & Answers about Il m'a dit que sa belle-sœur venait aujourd'hui.
Why use the imparfait venait instead of the present tense vient if she is coming today?
This is the rule of backshifting in reported speech. Since the main verb (a dit) is in the past, what he actually said at the time — Ma belle-sœur vient — must shift from the present tense to the imparfait (venait).
Why is there an m' before a dit?
The m' is short for the indirect object pronoun me. In French, the verb dire takes an indirect object for the person being spoken to, so you are literally saying he to me has said (Il m'a dit).
If we shifted the tense to the past, why didn't we change aujourd'hui to a past time marker?
In reported speech, you only change time words if the timeframe has actually passed. Because he kept aujourd'hui, it means the speaker is recounting this conversation on the very same day it happened! If he reported it a week later, he would have said ce jour-là instead.