Let  - The verb translated as let...alone" primarily means "to let go" or "to send away." This same word is usually translated as "leave," "forgive," "suffer," and "let" in the New Testament. This is a rare example of it being translated in its primary sense.

them -- The word translated as "them" is the Greek word commonly translated as third-person pronouns in English.  It is a plural, object.

alone: -- (CW) The concept of "letting go" and "dropping" is not the same as "letting alone."

they -- This is from the third-person, plural form of the verb.

be  - The verb phrase "they be" here is the common form of "to be" in Greek. It means to have a certain characteristic or remain in a certain condition.

blind  - (WP) "Blind" is a word that means both physically and mentally blind. It also means all things that are obscure. It is also a metaphor for disabilities of the other senses. This adjective appears before the verb not with the following now.

leaders  - (CW) "Leaders" is a noun, which means "guide" or "pilot."  It does not mean "leader" in the sense of a chief or head of a group. It is in the form of a subject of the sentence and immediately follows the verb "to be." This word is only used three times by Jesus.

of the blind. -- (OS) There is nothing in the Greek that can be translated as "of the blind" in the source we use today but it does exist in the source that the KJV translators used.

And -- (WW) The Greek word translated as "and" means "but," "however," and "on the other hand." It joins phrases in an adversarial way. Since it always falls in the second position, translating it as "however" often captures its feeling better.  

if -- (CW) The Greek word meaning "when" indicates more of an expectation of something happening than "if" alone. This is often how we use the word "when." It is not the word that simply means "if."

the -- (IW, WN) There is nothing that can be translated as "the" in the Greek source. Adding it makes the following word seem plural, but it is singular.

blind  -- "The blind" is a word that means both physically and mentally blind. It also means all things that are obscure. It has no article "the" before it. It is singular, not plural, so "a blind one" or "a blind person."

lead  - The word translated as "lead" is the verb form of the noun "leaders" used before. It means "to lead one upon his way," and "to guide." These concepts were also used for those who helped those ignorant of a given area. This word is only used three times by Jesus.

the -- (IW, WN) There is nothing that can be translated as "the" in the Greek source. Adding it makes the following word seem plural, but it is singular.

blind  -- "The blind" is a word that means both physically and mentally blind. It also means all things that are obscure. It has no article "the" before it. It is singular, not plural, so "a blind one" or "a blind person."

both  - The word translated as "both" means "both sides" and "both ways" as well as "both together." It is chosen because unlike the common word for "both," it implies two different ways or sides together.

shall -- This helping verb "shall" indicates that the verb is the future tense. Helping or auxiliary verbs are needed to translate the Greek verb forms into English.

fall  - "Shall fall" is translated from a Greek word that means "to fall" and "to fall down." It is the root word for dozens of Greek terms involving moving from a higher state to a lower one. Like our word "to fall" it has a number of special meanings including "to fall into a given class," "to prostrate," "to fall from power," "to perish," and so on.

into  - The word translated as "into" means "into" a place, "towards" as a direction, and "up to" limits in time and measure.

the -- (IW) There is nothing that can be translated as "the" in the Greek source.

ditch.  - "Ditch" is a Greek noun that means any hole or pit dug into the earth. It is also a term used to refer to graves and a pit from offering sacrifices.