Old, Ancient, Out-Of-Date
い adjective
This is a single kanji with an い on the end, meaning you know it's probably an adjective. What's the adjective form of old? It's also old, ancient, or out-of-date. This word is used for things like clothes, food and friendships, but not for people! So a 古い友人 (old friend) is someone you've been friends with for a long time, not an elderly friend.
ふるい
Since this word is made up of a single kanji with some hiragana attached, it's going to use the kun'yomi reading. You learned the on'yomi when learning the kanji portion, so let's use a mnemonic to help you with the reading of this word: If you're old, you're also full (ふる) of memories. And you're probably full of old secrets. And you're full from old people food.
このドレスは古いです。
This dress is old.
古いタブレットが九千円でうれた。
The old tablet sold for 9,000 yen.
アンティークとか古いものがすきなんです。
I like old things like antiques and whatnot.