Let us read two verses from the Qur'an, Surah Al Kahf (23-24 verse) and understand the significance of saying " اِنۡ شَآءَ اللّٰهُ ":
The background is the anecdote of People of Cave. The disbelievers asked the Prohet of Allah about the people of the cave one day. Since Prophet Muhammad ﷺ did not know the answer, he replied that he would answer tomorrow, knowing that he would get some Divine indication to the query of the disbelievers.
This plain answer was disliked by Allah and the Prophet ﷺ was straightaway admonished:
وَلَا تَقُوۡلَنَّ لِشَاىۡءٍ اِنِّىۡ فَاعِلٌ ذٰ لِكَ غَدًا ۙ
(18:23) And never say about anything: "I shall certainly do this tomorrow"
In its stead the Prophet ﷺ was advised that unless Allah wills a thing, nothing can move or happen. Let us read the next ayat and its explanation to understand why we should never commit ourselves to anything and why should we always say " اِنۡ شَآءَ اللّٰهُ " to include willingness of Allah. We must never rely upon our own resources so much as to forget Allah. If by any chance we do forget, we must come back to Him and keep Him in remembrance
اِلَّاۤ اَنۡ يَّشَآءَ اللّٰهُ وَاذۡكُرْ رَّبَّكَ اِذَا نَسِيۡتَ وَقُلۡ عَسٰٓى اَنۡ يَّهۡدِيَنِ رَبِّىۡ لِاَقۡرَبَ مِنۡ هٰذَا رَشَدًا
(18:24) unless Allah should will it. And should you forget (and make such a statement), remember your Lord and say: "I expect my Lord to guide me to what is nearer to rectitude than this."
This is a parenthetical clause which has been inserted here because of its relevancy to the preceding verse, in which it was asserted that the correct number of the Sleepers of the Cave is known only to Allah and a research into it is a useless task. Therefore one should refrain from investigating into unimportant things, nor enter into discussions about them. This has led to the instruction contained in the parenthetical clause for the benefit of the Holy Prophet and the Believers who have been told never to make a positive assertion like this: "I will do this thing tomorrow", for you do not know whether you will be able to do that thing or not: you have neither the knowledge of the unknown nor have full powers to do what you like. If ever inadvertently you utter anything like this, you should at once remember your Lord and say, "Insha Allah." Besides this you do not know whether there will be any good for you in the thing about which you say, “I will do this.” It is possible that you may do another thing better than that. Therefore you should trust in God and say, "I hope that my Lord will guide me in this matter with that thing which is nearer to the right way for me."
Tafsir Ibn-Kathir: Saying "If Allah wills" when determining to do Something in the Future
Here Allah, may He be glorified, shows His Messenger the correct etiquette when determining to do something in the future; this should always be referred to the will of Allah, the Knower of the Unseen, Who knows what was and what is yet to be and what is not to be, and how it will be if it is to be. It was recorded in the Two Sahihs that Abu Hurayrah said that the Messenger of Allah said:
(Sulayman bin Dawud (peace be upon them both) said: "Tonight I will go around to seventy women ﴿according to some reports, it was ninety or one hundred women﴾ so that each one of them will give birth to a son who will fight for the sake of Allah.'' It was said to him, ﴿according to one report, the angel said to him﴾ "Say: `If Allah wills'", but he did not say it. He went around to the women but none of them gave birth except for one who gave birth to a half-formed child.) The Messenger of Allah said, (By the One in Whose hand is my soul, had he said, "If Allah wills,'' he would not have broken his oath, and that would have helped him to attain what he wanted. ) According to another report, (They would all have fought as horsemen in the cause of Allah.)
(وَاذْكُر رَّبَّكَ إِذَا نَسِيتَ And remember your Lord when you forget) It was said that this means, if you forget to say "If Allah wills", then say it when you remember. This was the view of Abu Al-`Aliyah and Al-Hasan Al-Basri. Hushaym reported from Al-A`mash from Mujahid that concerning a man who swears an oath, Ibn `Abbas said "He may say `If Allah wills' even if it is a year later.''
Ibn `Abbas used to interpret this Ayah: (And remember your Lord when you forget) in this way. Al-A`mash was asked, "Did you hear this from Mujahid" He said, "Layth bin Abi Salim told it to me.'' The meaning of Ibn `Abbas' view, that a person may say "If Allah wills'', even if it is a year later, is that if he forgets to say it when he makes the oath or when he speaks, and he remembers it later, even a year later, the Sunnah is that he should say it, so that he will still be following the Sunnah of saying "If Allah wills'', even if that is after breaking his oath. This was also the view of Ibn Jarir, but he stated that this does not make up for breaking the oath or mean that one is no longer obliged to offer expiation. What Ibn Jarir said is correct, and it is more appropriate to understand the words of Ibn Abbas in this way. And Allah knows best.
Javed Ahmad Ghamidi Explanation:
Ie., when they test the Prophet (sws) by asking him such questions as who the dwellers of the caves were and what their story was, he should not promise them from his own self. It is possible that about a certain question the wisdom of God entails that an answer not be given or given later. For this reason, he should not make an unconditional promise by relying on divine revelation. In fact, all promises should be made conditional to the will of God.
Qur'an Wiki:
Every action a human being does or omits to do, indeed every breath a human being takes, is subject to God’s will. The curtains hiding the future are stretched in full so as to hide everything beyond the present moment. Our eyes cannot discern what is behind that curtain, and our minds are finite, no matter how advanced our knowledge may be. Hence a human being must never say that he is definitely doing something tomorrow unless he attaches his intention to God’s will. This is because tomorrow belongs to the realm that lies beyond the reach of human perception. As such, it is known only to God. Hence, we do not make any assertion about it.
This does not mean that man should be fatalistic, giving no thought to the future and making no plans for it. He should not live for the present moment, cutting himself off from his past and future.
