To be very clear on the subject we go through the single declinations to give you a better understanding.
Within the declination the subject of a sentence is called nominative. To find it, the question is who? It is the basic form of the noun or personal pronoun, without any changes.
Example
He
eats
a
cake.
Who
eats
a
cake?
He.
Il
mange
un
gâteau.
Qui
mange
un
gâteau?
Il (lui).*
They
eat
a
cake
Who
eats
a
cake?
They.
Ils
mangent
un
gâteau.
Qui
mange
un
gâteau?
Ils (eux). *
* Here we have a specialty in French, when asking for subject and answering with the personal pronoun, the accentuated personal pronoun is used. We will come back to this later.