that you may warn (people) thereby (and it be) a reminder to...
that you may warn (people) thereby (and it be) a reminder to the believers" "Follow what has been sent down to you from Your Lord....." This phrase is for the attention of those who are responsible and are also fully capable; it means that they should follow that which Allah has sent down to them. This 'act of following' concerns those affairs which are either obligatory, or recommended, or legitimate.
In these affairs a believer should follow the command of Allah*,* and the one must believe in them. One of its examples is that which says it is obligatory to avoid from forbidden things. ".... and do not follow any guardians other than Him...." The verse warns us not to follow anything other than the Qur'an so otherwise we will become sinners. It is evident that whoever is not the follower of the Qur'an is the follower of Satan and the idols.
That is why Allah commands us to follow the Qur'an and avoid following other than it. To follow the Qur'an is the same as to follow Allah*.* ".... how seldom do you remember!" This part of the verse addresses the pagans and those disbelieving people who rarely remember the Truth and little they take advice. In fact, the purpose of this phrase is to say that the style of their manner is not agreeable.
"And how many a town that We have destroyed, Our punishment came unto them by night or while they slept at midday." 5. "So their cry, when Our punishment came unto them, was not save that they said: 'Verily we were unjust '." In the previous verse, people were commanded to follow the Qur'an, and to leave out following anything other than the Qur'an. Now, this verse reminds them the fate of the past unjust nations so that they may fear and not to repeat the manner of those nations.
It says: "And how many a town that We have destroyed, Our punishment came unto them by night or while they slept at midday." Allah says that He has destroyed many people of the towns, and the Divine punishment was sent down over them by night or when they were relaxing at noon.