Mat 13:21 Yet he has no root in himself, but endures for a while: for when tribulation or persecution arises because of the word, by and by he is offended.

Here, the original Greek is actually closer to the way we say things today that the KJV, though it sounds less religious and more pragmatic.

Alternative version: But he has no root in himself, moreover he is an opportunist: but pressure and being hounded comes through the word and directly he is caused to stumble.

The first "but" in the original verse is not the normal preposition translated as "but" which is de, translated here as "yet" and "for when." This second "but" is alla, which means "but," nevertheless," "rather," "moreover," and "nay." It denotes an exception.

"Endures for awhile" is from esti (is) proskairos. This later word means "occasional," "temporary," "opportune," and "lasting for a time."

"Tribulation" is from thlipsis, which means simply "pressure," or "crushing," and is a metaphore for "oppression" and "affliction."

"Persecution" is from diôgmos, which means "the chase," "pursuit," and "harassment."

"Arises" is from ginomai, which we have seen many times before and means "to become," and "to come into existence."

"By and by" is from euthus, which we saw a in the last verse and means "straight," "direct," and "straight away."

"Offended" is from skandalizô, which means "to cause to stumble," "to give offense," and "to scandalize."