Yesterday – ayer
Yesterday I ran out of petrol. Ah! – if only I'd carried a jerrycan (ayer) of fuel in the back!
Note: The pronunciation of "y" in Spanish is not consistent. According to region or country, the sound may be like the "j" in "jug" (as in the mnemonic above) or like the "y" in "yes". However, whichever pronunciation you use, you will most likely be understood just as well. "Y" on it's own means "and", and this is always pronounced "ee".