Stressed syllables must end in a long vowel or a long consonant (just hold it!).
Mm is not used at the end of a word, so when a word ends in m it is not possible to tell if the preceding vowel is long or short.
Notes on the pronunciation of individual letters
A has a similar quality, whether long or short, to English father.
D is usually silent at the end of words and in the combinations ld and nd. However, d is pronounced in e.g.: Gud 'God', ned 'down' and sted 'place' and also if an r follows.
Eg is often pronounced as ei, e.g. meg 'me'.
Er is usually pronounced as the vowel in English the followed by trilled r, but in some words the vowel is more a-like, as if it were written ær: her 'here', der 'there', hver 'each', verst 'worst'.
Ei is midway between the vowel in day and dye; closer to the latter.
G is silent before j. e.g. gjøre - 'do' and in the adjective ending -ig. Ge is not pronunced in morgen 'morning'. G is pronounced as English y before i, e.g. gi - 'give'.
H is silent before j and v. e.g. hjerte 'heart' and hver 'each'.
J is pronounced as English y.
Ki, kj and tj are pronounced as the h in English huge (with more friction) or the ch in German ich, e.g. kirke 'church'.
O especially when long is u-like. For example, o in sola 'the sun' is, ironically, close the vowel in English moon.
R is generally trilled. However, it causes retroflex (tongue rolled back) consonants when it comes before dentals (d, n, t).
Note that faren, moren, broren, søsteren are all pronounced as if the e is not there, with retroflex ns.
Rs is pronounced as English sh even if the r ends one word and the s starts another e.g. norsk 'Norwegian' and Det er_sent 'It's late'.
Sk and skj are pronounced as the sh in English ship.
T is not pronounced in det 'it, that' and the neuter singular definite ending (-et) e.g. huset 'the house' (unless the genitive s follows: barnets 'the child's').
Tj see ki.
U is pronounced with more tension in the lips than in English, but not quite so much as German ü.
Y is like the vowel in English eat, but with narrowly rounded lips.
Æ similar to the vowel in Southern British bad.
Ø similar to the vowel in German Öl ('oil', not 'beer', as in Norwegian!) or, to a lesser extent, English sir.
Å similar to the vowel in English saw.
Words with unexpected pronunciations
Det 'it, that' is pronounced as if it were written 'de', when stressed like the first part of the English vowel in 'day'; when unstressed with the vowel in English 'the'.
De 'they; you (formal - always written with a capital letter)' is pronounced as if were written 'di'.
Halv 'half' is pronounced as if it were written hall.