Both words refer to a state in which all movement or progress has ceased. Standstill is used of both physical motion and more complex and abstract processes: The train came to a standstill; Negotiations are at a standstill. Impasse is a strong synonym for standstill in the latter usage: Peace talks have reached an impasse. However, it’s a more specific word, which suggests not only total lack of movement but also the impossibility of there being a way out or through. Standstill is often used of business, industry, the economy, and traffic, while an impasse is usually “between” two opposing parties and used of issues and discussions (the budget impasse; a political impasse). It’s common to speak of reaching an impasse, resolving or solving the impasse, or even breaking the impasse.
Both words refer to something that is readily apparent or easily perceived or sensed. As an adjective, clear has a wide range of reference: from materials (clear glass) to communication (a clear explanation). The relevant sense here describes something that is apparent, obvious, or easily perceived or understood: the team’s clear sense of pride. Palpable is a more emphatic, even dramatic word for something easily perceived, because of its literal meaning—able to be touched or felt—and its usual application to things like a mood, emotion, or atmosphere, which are insubstantial and can’t be touched. It suggests that an emotion or mood has such a strong presence that it is like something solid and material. The frequent use of the phrase almost palpable draws on both definitions of the word to make this point.
Both words refer to being keenly aware and attentive to one’s surroundings or the activity one is engaged in. Alert suggests being fully awake and aware and thus capable of quick perception and reaction: stay alert while driving; only alert readers noticed the error. Vigilant suggests a more consciously sustained watchfulness, for the purpose of detecting danger: we must remain vigilant until the threat has passed. It commonly modifies citizens, neighbors, and guards, as well as processes like enforcement, monitoring, and oversight.