The Spanish gerund (present participle) is formed using the ends -ando for -ar, -iendo for -er ad -ir.

Normally the verbs used to that form are estar, continuar and seguir, and verbs of motion when the fact is in progress in that moment.

This are the progressive tenses in Spanish:

Present progressive

Los niños están (siguen) jugando./The boys are (continue) playing.

Imperfect progressive

Mi perro estaba (seguía) comiendo./My dog was (kept) eating.

Conditional progressive

Tú estarías hablando todo el tiempo./You would be talking all the time.

Future progressive

Ellos continuarán diciendo lo mismo./They will continue saying the same.

Spanish gerund is often the equivalent of by + an English present participle.

Jugando, perderás mucho tiempo./ By playing you will lose many time.

In Spanish, people don't use present simple to describe the action that they are doing at the same time that is talked.

¿Qué haces? - Como fruta. (NO)
¿Qué haces? - Estoy comiendo fruta. (YES)
I eat fruit (I am eating fruit at this moment).

To do that, people use the present continuous. The use of the simple present in Spanish is normally for describing actions that usually Spanish people do.

Como fruta todas las mañanas./I eat fruit all the mornings.

Some -ir verbs change from -e- to -i- (sentir / sintiendo; vestir / vistiendo; repetir / repitiendo; sonreir / sonriendo; mentir / mintiendo; decir / diciendo ...)