under /ˈʌndər/ (US), /ˈʌndə/ (UK) from under the.
Five letters, none silent. Stressed /ʌ/ is spelled U, then the /nd/ cluster.
Final E R is reduced: /ər/ in America, /ə/ with a silent R in Britain.
Put the keys under the mat.
No doubled D, no A for U.
The dog is sleeping under the table. The U carries the stress, /ˈʌ/, so the whole word leans on that first syllable.
We sat under the stars until midnight. The final E R stays relaxed: /ər/ in American English, /ə/ with a silent R in British English.
Spelling and pronunciation traps
British English only: when under is final in a spoken phrase with a pause after it, the weak final /ə/ can lengthen toward an open /ɐ/-like sound, as in Put it under. Before more speech, as in under a table, the short weak /ə/ with its silent R stays.
US listeners hear that final R because American English is rhotic: /ˈʌndər/. UK dictionaries give /ˈʌndə/ because British English is non-rhotic here. Same spelling, both standard; pick one variety and stay consistent.