少ない and 少し: a small amount or doing a little
Distinguish the adjective 少ない, which evaluates a number or amount as small, from 少し, which expresses a small quantity, degree, or duration. Context also determines whether English needs few or little, or the more positive a little or some.
少ない: evaluate a quantity as small
少ない is an i-adjective used when the number or amount of people, objects, time, or opportunities is below a reference point. 今日は駅前を歩く人が少ないです。 and 荷物が少ないので小さなかばんで十分です。 commonly mark the item in short supply with が.
少なくない means not few or not little and can imply a substantial number in context. 参加者は少なくありません。 should not be read as saying that only a few participants came.
少し: a little, some, or briefly
少し modifies the quantity or degree of an action or state. 塩を少し入れてください。 asks for a little salt, while ここで少し待ってください。 asks someone to wait briefly.
It can also modify a noun with の, as in 少しの水. 少し can present a small quantity positively, so English may require a little or some depending on context. It does not inherently mean that almost nothing exists.
Use sentence structure to distinguish them
水が少ないです。 evaluates the amount of water as small, while 水を少し飲みました。 says that someone drank a little water. 少ない is the predicate in the first sentence, and 少し modifies the amount drunk in the second.
時間が少ないので少し急ぎましょう。 uses both forms. 少ない judges that little time remains, while 少し adjusts the degree of hurrying. Do not exchange them merely because both may be translated with little.
Quick check: Which belongs in 塩を（ ）入れてください to mean ‘Please add a little salt,’ 少ない or 少し?
Show the answer and why
Use 少し: 塩を少し入れてください。 It modifies the amount added. To state that the amount of salt itself is small, say 塩が少ないです。