Question

What is the subject

Be sure to lock the door before going to band practice

a band

b practice

c door

d (you)

Answer 1

each sentence is made up of 2 parts- subject/predicate (excluding simple/complex subjects and predicates [you'll learn that later on in 8th grade, and in high school] )
subject is a person place or thing (A NOUN).
a predicate describes what the noun is doing.
The subject of this sentence is "the door".
The predicate of this sentence is "band practice