"Comenzó hace tres semanas."
Translation:It started three weeks ago.
In this case "hace" means "ago". If it helps, you can think of the sentence as "it makes three weeks (that) it started". But obviously, more naturally, "it started three weeks ago".
Another interesting thing about "hace" is when it is used in weather conditions... hace sol (it is sunny), hace viento (it is windy). One theory is that this was used many, many years ago when people were talking about God. Él hace sol... Él hace viento. He makes sun... He makes wind.
When discussing when something happened, you say "hace [amount of time]" for example, "hace dos horas"
This is true for discussing how long ago something happened. "Hace dos horas" is two hours ago.
You can also use "desde hace" to discuss how long something happened; its duration. Estudié desde hace tres semanas: I studied for three weeks.
Why would "I started three weeks ago" be incorrect? How are you supposed to know I vs. It when it ends in "o"? Thanks.
"I started" would be comencé
ó at the end of a verb is 3rd person singular preterite (he/she/it/usted)
o without an accent is 1st person singular present tense
Duo is pretty bad about this, but an example like "it started three weeks ago" - it's probably "it" because we don't usually talk about people having started three weeks ago.
That being said, Duo should accept he/she/it/usted, since they all use the same conjugation.
There are three columns of uses of hacer! I cannot find the relationship to hace. It must be there because hace is not listed separately meaning ago.
Sentences need a subject. Technically, "she started" "he started" or "you (usted) started" would also be correct.
Comenzó = third person singular conjucation. "It" is included in the conjugation of the verb.
It's been three weeks since it started. =
Ha sido tres semanas desde que comenzó.
Three weeks back= tres semanas atrás. Maybe it is not as literal as Duo wants.