There are some basic rules to follow to decide when to use a capital letter and when to leave the first letter lowercase. Actually, maybe I should state that many rules in English capitalization, do not apply in Spanish. In Spanish you do not capitalize the names of the days of the week, or the months of the year.
Equations and Expressions are closely related. The primary difference between the two is an equals sign. An equation has a left side, a right side and an equals sign separating the sides. An expression, by contrast, doesn't have any sides and is simply what the name suggests: An algebraic expression.. Though sometimes it is possible to combine like terms, we are generally not expected to do or solve anything regarding expressions.
An expression is basically an incomplete mathematical sentence. It is like any normal phrase in the English language. Compared to expressions, equations are more complete. They are homologous to what completely structured English sentences are.