why is it not always necessary to include a subject pronoun before a verb in spanish. write answer in english and give an example in spanish

It's important to remember that in Spanish the verb forms often make subject pronouns unnecessary, and when that's the case the pronouns shouldn't be used unless there's a reason to.

tengo que limpiar

Des correcto.
las está pagando
porque esta hablando de una persona “el”

These objects represent mayan numerals. just like roman numerals, they are combined to symbolize a numeral and this enabled astronomers, scholars and architects to compute various equations yet it was also used by farmers and traders in their day to day transactions.