Before, ahead, forward are comparable when they mean in advance, especially in place or in time.
Before is more commonly used in reference to time than to place. Its most frequent implication is previousness or priority.
Sometimes, however, it implies futurity. This use is not a contradiction of the temporal sense, but an extension of the adverb in its less frequent meaning of in front or in the van.
Ahead and forward are the commonest adverbs indicating position in advance or in front of something and have practically supplanted before; ahead, however, usually implies a position outside of a thing, often a moving thing, and forward frequently implies a front position in the thing itself.
Thus, to send a group of scouts ahead implies their detachment from an advancing body of troops; to send a company forward usually means to send them to a position nearer the front or in the van of a regiment.
In nautical context ahead (opposed to astern) indicates a position or direction in front of and outside of the ship while forward (opposed to aft) indicates one in front of the midships section of the vessel.
The same distinction is often found in extended use; thus, one who looks ahead can foresee the remote consequences of a decision and ignore the immediate results, while one who looks forward anticipates something likely or bound to occur.
There is no difference between ahead and forward in reference to mechanisms which can be reversed or the opposite, except as determined by idiom.
Forward, unlike ahead, is rarely used in reference to time except in its sense of onward (see ONWARD).
Ahead, on the other hand, comes close to before in its implication of previousness or priority. Thus, a person finds himself forward of an appointment when he arrives ahead of time; idiomatically, one puts the clock forward (or sets it ahead) when he changes the position of the hands so that they record a later time.