Breakdown of Þetta er merki, svo maður verður að vara sig.
þetta
this
vera
to be
að
to (infinitive marker)
svo
then / so (therefore)
Questions & Answers about Þetta er merki, svo maður verður að vara sig.
Why is maður used here when the English translation says "you"?
The word maður literally means "man" or "person", but in Icelandic, it is very commonly used as the general, impersonal pronoun "one". While an English speaker usually says "you have to watch out" in a general sense, an Icelander says "one has to watch out" (maður verður að vara sig).
Doesn't the verb verða mean "to become"? Why does it translate to "have to" here?
You're right that verða usually means "to become" or "will be". However, when it is followed by að and another verb (here, að vara), the phrase verða að changes meaning to "must" or "have to".
Why is there a sig at the very end of the sentence?
It belongs to the verb! Að vara sig is a reflexive verb that means "to watch out" or "to beware" (literally, "to warn oneself"). Because the subject for this action is maður, which takes third-person grammar (like he/she/it), the matching reflexive pronoun is sig.
The English translation says "It is a sign". Why does the Icelandic use Þetta (this) instead of Það (it)?