Why write verbs ending ing and don´t put before the verb to be?
¿Por qué escriben verbos con terminación ing y no ponen antes el verbo estar?
Mi maestro de ingles de comento que el presente continuo se utiliza con el verbo estar, sin embargo en varias conversaciones he visto que utilizan verbos con ing sin utilizar esta regla.

Question

Why write verbs ending ing and don´t put before the verb to be?
¿Por qué escriben verbos con terminación ing y no ponen antes el verbo estar? 
Mi maestro de ingles de comento que el presente continuo se utiliza con el verbo estar, sin embargo en varias conversaciones he visto que utilizan verbos con ing sin utilizar esta regla.

Answer

"To be [verb]ing" is only one part of the grammar. In the [verb]ing form, which we call a present participle, you can also use [verb]ing as an adjective (eg. a cutting board), or at the beginning of a clause, (eg. scientists researching exotic diseases).

Also, if you add "ing" to a verb, it can also become a gerund. These look exactly the same as present participles, but they work in a completely different way, ie. as nouns.

No simple, single rule here.