Learn Russian: Мне страшно, потому что здесь темно.

In Russian, personal feelings and physical states are often expressed as things that happen to you rather than things you 'are'. This is called an impersonal construction. Instead of saying 'I am scared' using the nominative я, you use the Dative case мне (to me) followed by the state word страшно (scary). It literally means 'To me it is scary.'

Can I say Я страшный to mean 'I am scared'?

Be careful with this! If you use the standard adjective страшный with я, it describes your permanent characteristic. Я страшный means 'I am terrifying' or 'I am scary-looking.' To express the feeling of fear, you must use the Dative construction мне страшно.

Why do страшно and темно both end in '-о'?

When describing a physical or emotional state (like страшно) or a general state of the environment (like темно), Russian uses words that end in -о. These are called predicative adverbs or state words. They never change their ending to match a noun, because there is no subject noun in these impersonal sentences at all.

In the phrase здесь темно, where are the words for 'it' and 'is'?