Thanks to Jordi without whom this lesson wouldn't ever have been what it is now. The phonetics of one's native language is not the easiest thing to explain, one takes too many things for granted. There two single letters that represent two sounds, namely are z and x. They are useless in Finnish because syllable divisions fall between the compound consonants. They are usually preferred as ts and ks in spelling.
All vowels are pure vowels, and lax.
It duplicates the following consonant, it's pronounced as a glottal stop if followed by vowel or nothing. Nobody will notice any difference. As any language, Finnish has features that appear according to some systematical pattern, so they need not be presented in writing. Also there's no need to specify free variants in writing if they are understood similarly. It's something that a speaker does automatically, and thus learners may ignore.