While we're talking or writing, we have to add "a" or "an" before the thing when we mean "一個." So the question is how we know it's time to use "an" instead of "a." The answer is if the thing you're gonna describe begins with a vowel, you should add "an" before the noun. In other words, if the noun starts with a consonant, then you put "a" ahead.
當那個名詞是母音開頭時使用 a，當那個名詞是子音開頭時則使用 an．





So follow this rule. In this game, you have to do the grouping. There are 12 words. Six of them begin with a consonant, and they are Group A. The rest of them start with a vowel, they belong to Group An. You have to identify the words' group by choosing those in the same group together all at once. 有兩組，a組和an組．而我們要做的便是分別哪些屬於a組，哪些屬於an組．一次把屬於a組的選起來，再把an組的選起來．