The rule
German vowel length is not decoration — it separates words. The spelling tells you which it is: one consonant after the vowel (or an h) means long; two consonants mean short.
Long or short
| Käse · Brot · nehmen | long |
|---|---|
| Kasse · Butter · Suppe | short |
| der Staat / die Stadt | long / short |
Käse (cheese) and Kasse (till) differ only in that length. So do Staat and Stadt. Getting it wrong is not an accent — it is a different word.