You choose "a" or "an" based on the SOUND of the next word, not the letter.
1a. Open. Open starts with an "oh" sound, which is a vowel sound. An open door.
1b. One-day. One starts with a "wuh" sound, which is a consonant sound. A one-day course.
2. Euphemism. That starts with a "yoo" sound, which is a consonant sound. A euphemism.
Right now, I can't think of any words that start with A or I that start with a consonant sound, but E, O, and U both have a vowel sound and a consonant sound.
A one-time thing.
To summarize, consider how it sounds, not how it is spelled.
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