Matthew 24:32-35

“From the fig tree learn its lesson: as soon as its branch becomes tender and puts out its leaves, you know that summer is near. So also, when you see all these things, you know that he is near, at the very gates. Truly, I say to you, this generation will not pass away until all these things take place.  Heaven and earth will pass away, but my words will not pass away.

all these things - "The signs described in verses 4-28." (Ryrie)

There are several opinions on what, exactly, the Lord was referring to when He mentioned the "fig tree" (v 32) and "this generation" (v 34). I have copied the ones that make the most sense to me:

"The fig tree is the picture of Israel. The parable of the fig tree in Luke 13 is well known, and its application is Israel, to whom the Lord came, looking for fruit, and did not find it.Luke 21, the record there of this discourse, mentions likewise the fig tree and all the trees; these are the Gentiles, the nations. In Matthew 21, we see in the withered fig tree a type of Israel’s spiritual and national death. but that withered tree is to be vitalized. The fig tree will bud again. However, the characteristic of the fig tree is that fruit and leaves are there together. As soon as the branch becomes tender the fruit is found. It is a rapid development. This is the lesson here. Israel’s blessing, new life, fruit and glory will quickly be realized in those end days.. When in these last seven years, and especially the last 1,260 days, all these things come to pass, they will know that all which is promised to Israel will be at hand." (Gaebelein)

"One having passed through the rigors of a cold Judean winter would eagerly anticipate the coming of summer. When such a one sees the first green shoots appearing on a fig tree, he has an indication of the season in which he is then living (Matthew 24:32). He can anticipate the passing of the cold and the coming of spring. Christ applied this simple principle, saying, 'when you see all these things, you know that it is near, right at the door' (v 33). In the context 'these things' refers to the signs of verses 4-28. Those who will see the signs will know that He, the Messiah, or it, Messiah’s judgment, is at the door. Since these signs will all occur in the seven years of Daniel’s seventieth week, the generation that sees the beginning of these signs will 'not pass away until all these things have happened' (Matthew 24:34), for they all will fall within a brief span of time. These will not be signs given to a generation preceding the Rapture. Instead these signs will be given to a generation that cannot begin until after the church has been translated." (Pentecost)

"Some commentators refer 'generation' to the nation of Israel. The meaning, then, would be that Israel would continue as a nation until the second coming of Christ. Some take generation to refer to an indefinite period of time (age) and, accordingly, take it as instructing the disciples that the age leading up to the second coming will not end until the event of the second coming itself. A third explanation is that the word generation means what it normally means, that is, a period of thirty to one hundred years, and refers to the particular generation that will see the specific signs, that is, the signs of the great tribulation. In other words, the same generation that will experience the great tribulation will also witness the second coming of Christ." (Walvoord)

"No one living when Jesus spoke these words lived to see 'all these things' come to pass.  However, the Greek word can mean 'race' or 'family, 'which makes good sense here; i.e., the Jewish race will be preserved, in spite of terrible persecution, until the Lord comes." (Ryrie)

The most simple explanation — that the Lord was speaking of the generation of people who were alive when He spoke these words — cannot be true because the Lord’s second coming did not occur while they were alive.

The possibility that the Lord was wrong also cannot be true because His Word is truth (Jn 17:17).