The concept of the declining a verb is something which relates more to foreign languages than English; in Latin for example Amo means I love, amas means You love amat means he loves.

However some verbs are a lot more irregular

Sum I am
Es You are
Est He is

So in the series Yes Minister on British TV the joke was made that sometimes one can speak of irregular verbs as having a wider range of meaning: so in this case the same action done by mean has a very different meaning than if it is done by you or done by him...

Hope that makes sense; sorry for it being obtuse to you.