What factors affect language change? What causes a language to change over time?

Generation by generation, pronunciations evolve, new words are borrowed or invented, the meaning of old words drifts, and morphology develops or decays. The rate of change may vary from one place to another but whether the changes are faster or slower, they do happen and they happen for a good cause.

the present continuous is a verb tense which is mainly used to indicates that an action is in progress at the moment of speaking.  

it is formed using: person + verb to be (am/is/are) +verb + - ing form  

example: i am studying at this moment .

in addition, the present continuous is also used in order to express things that will or will not happen in the near future.

example: i am having dinner with some friends tonight.

the speaker sometimes   uses it   to express that an action is repeated and it causes irritation on the part of the speaker

example: my sister is constantly complaining about everything.