Difficult means needing a lot of effort or skill. A difficult question, a difficult journey, it is difficult to say. It is the standard word and it is neutral.
Hard is the everyday synonym and is commoner in speech; difficult is a shade more formal and is safer in writing, partly because hard has other meanings and difficult has almost none. There is one important extra sense: a difficult person is awkward and hard to deal with, and a difficult child is not one who struggles but one who resists. Note the construction it is difficult to do, and that difficult is never followed by an -ing form.
1adjectivedifficultNeeding a lot of effort or skill.
“It is a difficult piece to play well.”
2adjectivefullFull of problems or unpleasant to live through.
“It has been a difficult year for the whole family.”
3adjectiveawkwardOf a person, awkward and hard to please.
“He is being difficult about the dates.”
Too few real examples of this word to estimate which sense is commonest.
difficult is a degree of effort required.
Also accepted in games: hard, tough
Latin difficilis is dis-, not, plus facilis, easy — so difficult is simply not-easy, built the same way as impossible and inconvenient. The English word arrived through French and has never meant anything more complicated than its two halves.
Middle French
difficulter
Latin
difficultas
Old French
difficulté
Middle English
difficult
English, today
difficult
Built from
From Middle English difficult (ca. 1400), a a back-formation from from difficulte (whence modern difficulty), from Old French difficulté, from Latin difficultas, from difficul, older form of difficilis (“hard to do, difficult”), from dis- + facilis (“easy”); see difficile. Replaced native Middle English earveþ (“difficult, hard”), from Old English earfoþe (“difficult, laborious, full of hardship”), cognate to German Arbeit (“work”). The verb is from the adjective, partly after Middle French difficulter and its etymon Latin difficultō. Compare difficilitate, difficultate, and Italian difficoltare.
Etymology from Wiktionary, CC BY-SA