1. Not teaching you the right pronunciation
Swedish is a rhythmic language. There are many short syllables, and in each sentence, a few long syllables. A long syllable consists of a long vowel and a long consonant. Long means that it takes a long time to say.
In glas there is one long syllable, and this one is a long vowel - a.
In glass there is also one long syllable, and this one is long consonant - ss. It takes exactly as much time to pronounce glas as it takes to pronounce glass.
In glaset there is one long syllable, and one short [ _ . ]. The same applies to glassen [ _ . ]. In musik there is one short syllable, followed by a long one, i [ . _ ].
In a sentence, or phrase, only ‘interesting’ or ‘informative’ words have long syllables. Thus, jag vill ha glass still has only one long syllable, the ss in glass [ . . . _ ] and jag gillar musik [ . . . . _ ].
This needs to be taught from the very first moment of your very first Swedish class. Failure to pay attention to this fundamental detail of the Swedish language will lead not only to poor pronunciation but also to difficulties when understanding and using phrasal verbs, and for declining nouns in the plural.
And by the way, yes, it is possible for an adult to have an almost perfect pronunciation in Swedish, and, yes, this applies to anyone.
2. Not being picky about word order.
In all Latin languages, including, in this case, English, the first verb of any sentence must come after the subject. This is not the case in Swedish. The first verb always comes second, in any main clause. Students should be made aware of this from lesson 1, and they can start practising this also from the very beginning. Any mistake should be corrected, immediately. Errors in basic structure will make it impossible to learn more complex structures (for example sub-clauses) and this will stagnate complexity development on higher levels, as learners use the wrong word order automatically after only a short period of learning. For professions where language validation is necessary, a correct and complex structure is necessary to pass the exam, and if this has not been correctly from the start, it is very tiresome and sometimes impossible to correct fossilised erroneous structure.
Other grammatical mistakes should be corrected according to when the detail in question has been taught.
3. Not correcting your mistakes
Learners who are not corrected when mispronouncing a word or sentence or putting the words in the wrong order will be fearing of getting it wrong, and are thus discouraged to experiment with the language. Lack of correction will make learners shy and afraid of making errors.
4. Explaining why instead of showing how
Explanations are good and necessary, but teaching that focuses only on explaining why the language looks the way it does will make learners get a false impression of language being a theory. Teaching with a focus on explanation will result in learners waiting for the day when they will ‘understand it all’ and thus being ready to use the language. A good teacher shows how to construct a certain piece of language, explains the core of it briefly (in the target language. If they use another language, this is a clear sign that the explanation is taking too much space) and then spends at least five times as much time on making the students produce the same structure, and corrects it. This way, learners will make progress at every class.
5. Not using a natural language when communicating
Speaking slowly, using fragmented sentences, or pronouncing silent consonants will not help a learner to speak correctly, nor to understand spoken language.