Long vowels are something you’ll run into a lot and can be kind of difficult for English speakers, since this isn't something we have. ^^ Romaji of long vowels can also be confusing to read, so this is going to be a card teaching you how to pronounce it when there is a long vowel. It's important to know where there is a long vowel, because it can sometimes change the meaning of the word completely if said incorrectly.
A few examples of words that have long vowels are:
いい (ii) which means good. It is pronounced as "ee" like in "bee." Instead of pronouncing the i separately twice, you would hold the sound a little longer than usual, but be careful not too hold it for too long.
どうして (dou shite) which means why. Pronounced "doh she teh" the "o" in do is extended slightly because of the "u" sound.
Underneath is a video, it's in all Japanese with subtitles, she pronounces different words with long vowels.
Mother- okaa san - おかあさん
Father- otou san - おとうさん
Elder brother- onii san - おにいさん
Big- oo kii - おおきい
Card- kaa do - カード
Elevator- ere bee taa - エレベーター
Grandmother- obaa san - おばあさん
Aunt- oba san - おばさん
Grandfather- ojii san - おじいさん
Uncle- oji san - おじさん
Advanced- kou tou - こうとう
Thing- koto - こと