Hiragana is one of three Japanese alphabets. It is used to form words which (1) have entered the language since the adoption of Chinese kanji in the 7th century, and (2) are not foreign imports (these being formed from the 3rd alphabet: katakana). Transcribed into Roman alphabet (Romaji), hiragana characters almost invariably comprise two letters, the second of which is a vowel. For instance ひ is transcribed as 'hi' (the i pronounced as in 'if'), ら pronounced 'ra', が read as 'ga', and な as 'na'. Together these create ひらがな, or hiragana.
The two-letter pairings and final vowel sound in most hiragana characters often make Japanese sound a little staccato, which brings me back to the word for warm: atatakai. Hard enough to get right in its simplest form (above), pronouncing it it the negative and past is sort of like trying to pronounce supercalifragilisticexpialidotious when you first watch Mary Poppins: the harder you try to say it the further away you get, until you give up, laughing and somehow floating near the ceiling.
My teachers ask about the weather a lot, and so I try to tell them that it is:
あたたかい - atatakai - warm
あたたかくない - atataka kunai - not warm
あたたかかった - atataka katta - warm
あたたかくなかった - atatakaku nakatta - not warm.
Yesterday I correctly pronounced atatakaku nakatta for the first time. I'm still quite glad that now, when the teacher asks me what the weather is like, I can say, consistently: さむいです - samui desu - it is cold.