be, feel, look, seem
The chairs felt hard and uncomfortable.

become, go
Don't leave the cake uncovered or the icing will go hard.

stay

extremely, fairly, very, etc.
a little, slightly, etc.

rock hard
The toffee was rock hard.

be, look, seem
become, get
Life got very hard.

make sth
If you tell the children the answers, it only makes it harder for them to do the work on their own.

find sth
I found the exam quite hard.

extremely, fairly, very, etc.
a little, slightly, etc.

Hard is used with these nouns:
bargaining, bench, biscuit, blow, bread, cash, cement, chair, cheese, choice, cider, climb, coal, contact lens, copy, core, court, cover, currency, data, day, decision, disk, drinker, drive, drug, earth, effort, evidence, exercise, face, fact, fight, fighting, floor, foul, freeze, frost, gaze, graft, ground, hat, heart, jab, job, journey, kick, kiss, knock, labour, landing, lesson, life, line, liquor, look, luck, lump, man, margarine, mattress, mouth, muscle, news, nipple, palate, part, pavement, penis, punch, push, question, race, rain, reality, road, rock, sand, science, season, shell, shove, skin, slap, smack, snow, stone, struggle, surface, tackle, task, thigh, thing, think, thinking, time, toil, tone, training, truth, tug, water, way, wear, week, winter, wood, word, work, worker, workout, world, year