The line that is scanned incorrectly is line 1. This is because all the rest of the lines have 4 metrical feet, while the first one has two metrical feet.
The little straight line over syllables is called a macron. It is placed above stressed syllables.
The little curved line over syllables is called a breve. It is placed above unstressed syllables.
The correct way to scan line 1 would've been:
Two (macron) roads (breve) di (breve) verged (macron) in (breve) a (breve) yell (macron) ow (breve) wood (macron).
Putting these symbols over syllables is called scansion: The process of analyzing the meter of a poem.
Metrical foot: A unit of stressed/unstressed syllables that join with other feet to create poetic meter.
There are four different metrical types of feet:
iambic --> da DUM, da DUM, da DUM (3 iambic feet)
trochaic --> DUM da DUM da DUM da (3 trochaic feet)
dactylic --> DUM da da, DUM da da, DUM da da (3 dactylic feet)
anapestic --> da da DUM da da DUM da da DUM (3 anapestic feet)
Hope this information clears it up a bit! :)
I tried to put in an image, but it wouldn't let me for some reason. So I hope you understand what the correct way to write line 1 is!