"Next, go straight along the narrow path."
I picked the boxes in the following order and Duo said it was correct: つぎ、ますぐに... So i assume つぎ doesn't necessarily require a particle to be used correctly. Since both つぎ and ますぐ can be followed by に (one as a time particle, the other as a direction particle) but that Japanese people tend to avoid using the same particle twice in a single sentence (that's more of a feeling than a rule I guess) maybe both positions for に are acceptable and you just skip the other?
Because 狭い (せまい) is an -い adjective. As a rule, -い adjectives do not require a -な suffix when used before a noun, just their basic -い suffix. There are two Important exceptions: the adjectives for big (大きい or 大きな) and small (小さい or 小さな) are considered to have BOTH an -い form AND a -な form, and it is considered more correct to use the -な form when placing these adjectives before a noun. In most other uses, you use the -い form with these adjectives.
Not without changing the meaning. 行く is an intransitive verb meaning “to go to a location,” and you must use it with the participles に or へ, not を. So 道にまっすぐ行ってください would translate as “go straight TO the street,” with the street being the final destination, rather than “go straight ALONG the street,” which is the meaning with を…進む.
で is the
context particle, able to describe an action taking place at a given location, so it is often translated "at" or "by means of."
を as the
direct object particle, when applied to a direction verb, implies doing something directly on that location—usually translated as "through" or "along."
Understanding this, compare:
Next, go straight (along/down) the narrow path.
Next, go straight (at/on) the narrow path.
I suppose it can be understood similarly in English, but there are finer nuances here to consider for Japanese.