question pertaining to this exercise

Question

Hi, I have a question concerning this text:

"at the beginning of the 17th century,"

a comienzos del siglo XVII,

why is the plural used here and not the singular?

What rules/lesson pertain to this?

Thank you, NIcole

Answer

Hola Nicole
You can say:

"a comienzos de..."
"a principios de..."
"a inicios de..." (less usual)

or also:

"al principio de..." 
"al comienzo de..."
"al inicio de..." (less usual)

These last options with their singular form, not their plural form.
These are time phrases that are used this way in Spanish. Using the structure with "a + plural" sounds slightly more formal than the "al + singular", but they are both used a lot.
Saludos
Inma