How it feels (трудно, легко, известно)

The Impersonal Judgement

Russian judges a situation with a short adverb and no subject at all: Трудно понять. — It is hard to understand. The experiencer, if named, goes in the dative: Мне трудно понять.

predicativejudgement
трудноit is hard
сложноit is complicated
легкоit is easy
невозможноit is impossible
известноit is known
It is hard for me to talk about this.
Мне трудно говорить об этом.

Трудно or Сложно?

Трудно is about effort — the thing costs you strength. Сложно is about structure — the thing has too many parts. A heavy bag is трудно to carry; a bureaucratic form is сложно to fill in. In everyday speech they overlap, but the distinction is felt.

This problem is complicated to explain quickly.
Эту задачу сложно объяснить быстро.

The Evaluating Row

The same pattern carries evaluation: ужасно (terribly), смешно (it is funny), спокойно (it is calm), неплохо (not bad). They work alone, with a dative experiencer, or as intensifiers before an adjective — ужасно интересно is "terribly interesting", and the terror is a compliment.

Здесь спокойно. — It is calm here. Неплохо для начала! — Not bad for a start!

Известно

Известно — "it is known" — is the impersonal citizen of academic prose: Как известно, ... (as is well known ...). It also makes the personal construction Мне это известно — I am aware of that.

As is well known, it is cold here in winter.
Как известно, зимой здесь холодно.

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