(لَوْ كَانَ عَرَضًا قَرِيبًا وَسَفَرًا قَاصِدًا لَّاتَّبَعُوكَ) (Had it been an easy gain and a moderate trip, the hypocrites would have followed you): The Arabic word translated to ‘gain‘ means the spirit or essence of something. Thus, it refers to something which is either temporary or changeable. Allah is saying here, ‘If it were an easy matter and a journey without hardship, the hypocrites would have followed you and gone out with you to Tabuk, Oh Messenger of Allah.’

وَلَٰكِن بَعُدَتْ عَلَيْهِمُ الشُّقَّةُ ۚ  (but distant to them was the journey.): But to the hypocrites, there were all the difficulties; the long distance between Al-Medina and Tabuk, the intense heat, and it was a journey full of hardship.

(وَسَيَحْلِفُونَ بِاللَّهِ لَوِ اسْتَطَعْنَا لَخَرَجْنَا مَعَكُمْ) (And they will swear by Allah, “If we were able, we would have gone forth with you”): They did not swear yet, “will” indicates a future expected action. If they had understood, they would not have sworn, but Allah (swt) challenges them that in the future, they will indeed swear, and they did.

(يُهْلِكُونَ أَنفُسَهُمْ) (destroying themselves): So, they were causing their own destruction, not only because they lagged behind the Messenger of Allah (saws) in the Tabuk battle, but also because they falsely swore by Allah (swt). They were capable of joining but hesitated to support the Messenger of Allah (saws) due to fear of heat, fatigue, and hardship.