"Az én lovam nagyon lusta."

Translation: My horse is very lazy.

[scotthensman]

What's the "én," for?

[vvsey]

It stands for "My". With a possessive structure like this, with a pronoun, the pronoun stays in its nominative form, only the possessed gets a suffix.
My horse - az én lovam
Your horse - a te lovad
His/her horse - az ő lova
Our horse - a mi lovunk
Your horse - a ti lovatok
Their horse - az ő lovuk - yes, "ő", not "ők", even the plural "-k" is omitted from the pronoun.

[Arcaeca]

Isn't it not necessary, though? Why add the "én" if "A lovam nagyon lusta" suffices?

[vvsey]

Of course, it is optional. It is there for emphasis only.

[PetiM44]

It means "I", but it's not necessary. "A lovam nagyon lusta" would mean the same.

[Tielbert]

Why lovam, and not lóm?

[mizinamo]

Historical reasons.

The root was lov-.

In the nominative, where there is no final vowel, lov was considered not pronounceable, and so the -v disappeared while lengthening the vowel and you have ló.

In other cases, where the -v is not at the end of the word, it was kept.

Similarly with kő "stone" (accusative követ; compare Finnish kivi "stone").

[Tielbert]

Köszönöm szépen, very nice explanation

[RobertEddy]

So lazy it stopped for a nap on the way to Debrecen.