Mat 5:8 Blessed are the pure in heart: for they shall see God.
Lucky [are] these spotless in the heart, because they themselves the divine are going to see for themselves.
Here, the forms of the words cannot be translated as either "pure in heart" or "pure of heart". There is no active verb but both words are in the form of subjects. We have to assume this was spoken, where an "are" is implied, but possible as part of a dialogue.
The sense of the Greek word translated as "blessed" shifts its meaning among "happy", "wealthy", and fortunate depending on the context. "Fortunate or lucky seems to work here. This word only means "blessed" in the sense that the lucky are blessed by good fortuned. It is not a religious term.
There is no "is" here. It is added to make a written sentence as opposed to a spoken phrase.
The Greek word translated as pure means free from that which corrupts or soils. There is a hidden connection to the previous verse, Mat 5:7. The merciful are those that forgive debts and one meaning of "the pure" is someone who is debt-free. Though it is hidden, in the Greek these two beatitudes can be seen the two sides of the same coin.
The word translated as "heart" means the physical heart and the seat of emotions. The form as an indirect object, singular heart. The form could have two uses. First, it could describe an area affected, "in the heart". This works better with an active verb, but it could work. It could also indicate an instrument or means, "by the heart." Jesus consistently uses the Greek term to refer to the heart as the seat of emotions, so the sense is that we are purified by our emotions. In Greek, these were the strong, passionate, higher nobler emotions, "of the chest", rather than the lower, based passions "of the belly".
Again, the use of the pronoun for the subject "they" emphasizes it, since the pronoun is already part of the verb ending.
The Greek word for "see" is not the common word translated as see, but a special one, that primarily means seeing with the eyes or using to eyes, for example, to aim at something. The base of this word is the Greek term for the eye. However, it is also a metaphor for mental perceptions, but perceiving with the mind and to experience. This is the term used to refer to seeing visions. The form of this verb acts on or for itself. So "perceiving for themselves" or "envisioning for themselves."
There are a number of patterns in the Beatitudes, which are discussed in this article on the Beatitudes.
The form of the word heart can indicate a place of purity
The Spoken Version:
The smiling teacher quickly made his way toward a group of prostitutes. They were seated near the tax collectors and other detestables. Two of these women were clearly pregnant.
“Lucky!” The teacher announced, taking these soon-to-be mothers by the hand. He had them stand so the crowd could see them. The women blushed. “The spotless!” He said.
While many laughed, others registered their objections.
“in the heart!” The speaker amended touching his own heart. “Because they themselves?" He laid his hands on their large bellies and said, The divine are going to perceive for themselves.”
οἱ καθαροὶ (adj pl masc nom) "Pure" is from katharos, which means "physically clean", "spotless", "clear", "pure (water)", "clear of objects", "free of contamination", "clear of debt", "genuine", "pure of birth", "without blemish," and "sound."
τῇ καρδίᾳ, (noun sg fem dat) "In heart" is from kardia, which means "heart (the physical organ)", "the seat of emotions (especially passion, rage, and anger)", "inclination", "desire," "purpose", "mind", "the pith (in wood), and "the deep (of the sea)."
αὐτοὶ (adj pl masc nom) "They" is from autos, which means "the same," and the reflexive pronouns, "myself", "yourself", "himself", "herself", "itself," or the oblique case of the pronouns, "him", "her," and "it." It also means "one's true self," that is, "the soul" as opposed to the body and "of one's own accord."
ὄψονται (3rd pl fut ind mid) "Shall see" is from optanomai, which means "to see", "to look", "to aim at", "to look towards", "to have sight", "to take heed," (in transitive) "to behold", "to perceive", "to observe", "to look out for," and "to be seen (passive)." It is a metaphor for mental sight, "to perceive", "to discern", "to see visions", "to appear in visions (passion), and "to interview."