Christ is creating a series of contrary statement using the various meanings of the same word, the Greek term translated as "Blessed." When he referenced the poor in Mat 5:3, he implied the sense of the word as "wealthy." By talking about crying, he is referencing its other meaning as "happy."

The word translated as "they who mourn" is a participle of the the Greek verb meaning "to wail" and "to lament." This word is closely associated with the idea of mourning. The word is plural and masculine, but in Greek, as in English, the broad usage is masculine. The specific usage is feminine.

The "for" here is a causal adverb that means "seeing that", "because", or "since."

The "they" is used explicitly as the subject of the final phrase. This is unnecessary in Greek because the subject is also a part of the verb ending. Christ only uses the pronoun when he wants to emphasize it.

The Greek word that translated as "comfort," literally means will be "to be called near." So it primarily means "to be summoned" since it is in the passive. It also means "to demand," ("called from") "to encourage," ("called along") and "to excite" (called beyond"). We will see this word again when it is translated as "to call," "to beseech," and "to pray", (Mat 18:32, Mat 18:29, and Mat 26:53). In all of them, "call upon" works best in English both in the sense of "summoned" and "to ask."

What are mourned called upon to go or do? Since the topic is mourning, we can only assume that mourner are being summoned "beyond" this world, the realm that is "near." Both "near" and "beyond" are the basic meanings of the prefix of the "called near" verb.