труднее

In a sentence

• Эта задача гораздо труднее, чем я ожидал.

  This task is much harder than I expected.

  Here, 'труднее' is the comparative form of 'трудный' (hard/difficult), used to compare the difficulty of two tasks or situations.

• Любопытно, почему второй уровень оказался труднее первого.

  It's curious why the second level turned out to be harder than the first.

  'Труднее' is again being used as a comparative adjective to describe the relative difficulty between levels.

• Становится труднее сосредоточиться поздно вечером.

  It becomes harder to concentrate late in the evening.

  'Труднее' describes increasing difficulty over time, comparing it with earlier states of concentration.