I know this is long but read at least until the first point where i indicate it's fine to stop reading. It's only a paragraph or two.
Everyone else is pretty much right here, but I want to highlight how the present tense is used versus how the gerund is used. Rarely in my experience with spanish do you say "I am doing something." Spanish speakers use what are called "active verbs" far more often in speech than do english speakers, which is considered, even is English, to be more proper. This is especially true when writing, so there's a very important lesson to be learned from spanish and hispanic cultures when they make the distinction between the gerund and present tense.
A classic example would be how we would say:
"Watch out! A car is coming." when crossing the street with a compadre.
A spanish speaker would say:
"¡Ten cuidado! ¡Viene un coche!" which directly translates to "Take care (how they would say watch out)! A car comes!" rather than "a car is coming." They would not say "Un choche esta viniendo."
What I mean by "active verb" is that the spanish speaker uses the verb "to come" rather than a form of the verb "to be" followed with the gerund of another verb. This prevents the use of two verbs when one would suffice (the use of only "to come" rather than "to be" and "to come"), which makes the speaking far more efficient, as the listener learns the same amount of information in less words.
-That's all you really need to know but keep reading for relevant Spanish Quick-Tips™.
That concept of active versus passive verbs is translated into a verb form for use in the past tense, one you may have heard of, called the imperfect. First off, I want to say that whatever your textbook says about imperfect is correct, but take this advice and it will helps you understand a bit better. Here it goes:
The imperfect is used like your textbook says, but is often used like this as well (which i bet your textbook will not tell you):
the imperfect is a translation of inactive verbs into the past tense in order to make it more efficient. This is best demonstrated through examples.
Here it goes:
Comía helado.== I was eating ice cream.>>Notice how in spanish one verb is used but in english we, quite inefficiently, use two ("was" and "eating").
You would not say Estaba comiendo helado.==I was in the state of eating ice cream. (You could, but it's better not to. Even though the translation sounds kind of weird, it is much more acceptable to use inactive verbs in the past tense.)
Also very important: when saying "I like to do something" they use the infinitive rather than gerund. For example while we say "I like eating cookies," they say "I like to eat cookies." (Me gusta comer galletas.) "Playing soccer, cooking food, or crossing the street makes me tired." Me cansa jugar el fútbol, cocinar o cruzar el calle.==direct translation==To play soccer, to cook, or to cross the street tires me. >>>better translation:: "It tires me to play soccer, to cook, or to cross the street."
As far as I know, the only time the gerund is used in spanish is if the word "to be" (estar) or, and this is more my point, the words "while" or "by" can be substituted in front of it. Ex. "Limpiando el piso, se cansa."==(By) Cleaning the floor, he tires himself. Cruzando el calle, se pegó por un coche. (While) crossing the street, she was hit by a car.----stuff like that
Keep reading further still to become a Spanish Pro™
Ok i wrote this as a note to myself a long time ago and am kind of late so i don't have time to gear it towards you but I figure since i just have to cut and paste i might as well put it here.
Imperfect also makes since with the whole “cuando era niño...” thing. EX: Cuando era niño, comía el helado. That means “when I was young I used to eat (was eating) ice cream.” “Comía” literally means “was eating,” so in english, it would be something like “at the time (the time of childhood) I was eating ice cream.” It means “used to eat” if you don’t continue the clause/sentence. If you do continue the clause it changes the meaning, for example “Cuando era niño comía helado cuando, por accidente, dejé el helado al piso.”