Strive -- "Strive" is from a Greek verb that means to "content for a prize",  "fight", "content in court", and passively, "to be decided by contest".  It is in the imperfect past, referring to something started in the past but not completed.

to -- This "to" is added to create the infinitive form of the following verb.

enter -- "Enter" is a word that means "enter," "go or come into" of an accuse "come into court," and has the double meaning of "coming into one's mind."

in --  (WW) The preposition translated as "in" with the form of object here means "through,"  "over," " "in the midst of," or "by (a cause)." It indicates movement through a place or time, but it also means "by" the sense of "by means of" a given method.   This word doesn't mean "in." 

at -- (IW) This word is not in the Greek source.

the -- The word translated as "the" is the Greek definite article. The Greek article is much closer to our demonstrative pronouns ("this," "that," "these," "those") than the English "the." See this article for more. 

strait  - The term translated as "strait" primarily means "narrow," but it is not a positive term in Greek having many negative associations including "small-minded" and "confined."

gate:  - The term translated here as "gate" means specifically one side of a double gate that served as the entry to a town or large building. It was also frequently the location of a toll booth, changing a tax for entry into a town. The term also meant special doors or gates such as those to the women's quarters or the gates of hell. These gates were a point of control and authority.

for  - The Greek word translated as "for" acts as a causal adverb meaning "for what," "because," "since," and "wherefore."

many, -- The word translated as "many" means many in number, great in power or worth, and large in size.

I -- This is from the first-person, singular form of the verb.

say -- The word translated as "say" is the most common word that means "to say," "to tell," and "to speak,"  but when used with an objective noun or pronoun, the sense is "say of" or "speak of."    It also has many ancillary meanings such as "to count" ("to number" or like we might say, "to recount" a story) or "to choose for yourself."

unto -- This word "to" comes from the indirect object form of the following word that requires the addition of a preposition in English. The most common is a "to" for the English indirect object.

you, -- The Greek pronoun "you" here is plural and in the form of an indirect object, "to you," "for you," etc.

will -- This helping verb "will" indicates the verb is in the future tense. Helping or auxiliary verbs are needed to translate the Greek verb forms into English. - However, the verb could also be in the form of possibility, "might."

seek -- The Greek verb translated as "seek" has a variety of meanings around the idea of "searching" and "desiring." It has a sense of seeking with a specific aim. It also means to "search," "inquire into," "investigate, "require," "demand," and "examine."

to -- This "to" is added to create the infinitive form of the following verb.

enter -- "Enter" is a word that means "enter," "go or come into" of an accuse "come into court," and has the double meaning of "coming into one's mind."

in, -- (IW) This word is not in the Greek source.

and -- The Greek word translated as "and" is used as the conjunction "and," but it also is used to add emphasis.

shall -- This helping verb "shall" indicates that the verb is the future tense or a form that indicates possibility at some time. A "might" or "should" could work here or nothing at all in an "if/when/whoever" clause which implies a possibility.  Helping or auxiliary verbs are needed to translate the Greek verb forms into English.

not -- The Greek word translated as "not" is the Greek negative used to deny objective facts, not opinions. It means "no," "not," or"no truly." It makes a negative statement of fact. When a negative precedes the verb, it affects the whole clause. When it precedes other words, its force is limited to those words. 

be able. -- The word translated as "be able" is a verb that means "to be strong", "to be able," or "to have powerful." It is the present plural participle of verb, used as the sentence's subject.