As Millie said. I will add an extra comment: It has to do with the sound, not with the actual spelling. If it sounds like a vowel starts the word, put 'an', and if it sounds like it is a consonant, put 'a'. For example: A university (not 'an'). Why? Because 'university' is pronounced 'youniversity'. It sounds like it starts with a 'y' Versus - an ulcer - (not 'a') - it sounds like 'olser', starting with a vowel sound 'o' An hour (not 'a'). Why? Because in this case the 'h' is silent, and it is pronounced like 'awr'. It sounds like it starts with an 'a'. Versus: A horse (not 'an') - In this case the 'h' is pronounced, so it follows the regular rule for words starting with consonants

If the word after "a/an" starts with a vowel (A, E, I, O or U), you use "an." If it begins with a consonant (any other letter), you use "a." For "an insecure place," you use "an" because the word after "an" starts with a vowel, but it refers to "place" (the noun) rather than "insecure" (the adjective). You would say "a very insecure place" because the first letter of the word after "a" is a V which is a consonant. For the second one, you say "a pizza" because the word after the "a" starts with a consonant.