The word(s) "hard" appears 35 time(s) in 32 verse(s) in Quran in Abdullah Yusuf Ali translation.

(11) O Prophet! strive hard against the unbelievers and the Hypocrites, and be firm against them. Their abode is Hell,- an evil refuge indeed.

(12) Relate to them the story of Noah. Behold! he said to his people: "O my people, if it be hard on your (mind) that I should stay (with you) and commemorate the signs of Allah,- yet I put my trust in Allah. Get ye then an agreement about your plan and among your partners, so your plan be on to you dark and dubious. Then pass your sentence on me, and give me no respite.

(13) Moses prayed: "Our Lord! Thou hast indeed bestowed on Pharaoh and his chiefs splendour and wealth in the life of the present, and so, Our Lord, they mislead (men) from Thy Path. Deface, our Lord, the features of their wealth, and send hardness to their hearts, so they will not believe until they see the grievous penalty."

(14) When Our Decree issued, We turned (the cities) upside down, and rained down on them brimstones hard as baked clay, spread, layer on layer,-

(15) For them is a penalty in the life of this world, but harder, truly, is the penalty of the Hereafter: and defender have they none against Allah.

(16) Remember! Moses said to his people: "Call to mind the favour of Allah to you when He delivered you from the people of Pharaoh: they set you hard tasks and punishments, slaughtered your sons, and let your women-folk live: therein was a tremendous trial from your Lord."

(17) And We turned (the cities) upside down, and rained down on them brimstones hard as baked clay.

(18) "Or created matter which, in your minds, is hardest (to be raised up),- (Yet shall ye be raised up)!" then will they say: "Who will cause us to return?" Say: "He who created you first!" Then will they wag their heads towards thee, and say, "When will that be?" Say, "May be it will be quite soon!

(19) That He may make the suggestions thrown in by Satan, but a trial for those in whose hearts is a disease and who are hardened of heart: verily the wrong-doers are in a schism far (from the Truth):

(20) Or (the Unbelievers' state) is like the depths of darkness in a vast deep ocean, overwhelmed with billow topped by billow, topped by (dark) clouds: depths of darkness, one above another: if a man stretches out his hands, he can hardly see it! for any to whom Allah giveth not light, there is no light!