If you learn a foreign language, it's very important to know how to describe a human condition. We should be able to say whether we are fine or bad, warm or cold in our speech. It's a very important skill - the skill to describe your own (or another person's) physical condition or the state of mind. To say how a person feels himself, in Russian we use the construction "a noun in Dative + verb "быть" [byt'] (to be) + adverb".
If we talk about condition or mood we have at the moment, we use the verb "быть" (to be) in zero form (in other words we just omit it). In the case when we describe a human condition in the past tense, the verb "быть" transforms into "было" [byla] (was/were), in the future tense - "будет" [budit] (will be). Pay your attention to the endings of verbs and adverbs in the past tense - it can help you to memorize the construction better.
Было хорошо [byla kharasho] - I felt good, было весело [byla v'esila] - I had fun, было тепло [byla tiplo] - it felt warm.
Adverbs help to describe mood and the state of mind (hereafter they are presented as antonyms):
хорошо [kharasho] - плохо [plokha] (good - bad)
весело [v'esila] - грустно [grusna] (funny - sad)
интересно [intir'esna] - скучно [skuchna] (interesting - boring)
легко [likhko] - трудно [trudna] (easy - difficult)
тревожно [trivozhna], страшно [strashna], жутко [zhutka] - спокойно [spakoyna] (anxiously - calmly)
Physical condition can be also described with adverbs:
жарко [zharka], тепло [tiplo] - холодно [kholadna], зябко [zyapka] (hot/warm - cold/chilly)
больно [bol'na] - приятно [priyatna] (painfully - nice)
Мне было грустно одному - I was upset being alone.
Мне и моим друзьям весело (present tense, verb "быть" in zero form) - We have fun with my friends.
Ученикам будет интересно в школе - It would be interesting for students at school.
Ему было жарко на пляж, - It was very hot for him at the beach. В горах нам будет холодно - We will be cold in the mountains.