The English language has a total of 15 vowels. In writing the vowels, I will use their IPA representation. They consist of the front vowels: i as in eat, week, and tree. I as in pin and invite. e as in eight, locate, and ballet. as in extra and bet. as in at and glass. and a as in the British pronunciation of path. Our central vowels are: as in under and sun. as in asleep, balloon, and zebra. as in herd, earth, fur. as in farmer and waterfall. Back vowels are: u as in drew. as in foot. o as in over, boat, hello. as in all, hall, jaw. as in awkward, clock, and raw. There are three diphthongs which include: as in Iaisle, mine, and try. as in hour, mouth, and plow. and as in Ioil, spoil, and destroy.
In comparison, the Spanish (Mexico) language has 5 vowels. A, E, I, O, U. They are consistent in every word. They always have the same sound.
There is a reason that English is a difficult language to learn. Not only do we have a plethora of vowels, but they are not consistent in how they are pronounced. For example, the word "bow" can be pronounced as /b or /bo/. The 'ow' in this word can represent more than one vowel. So, when our children are struggling to learn this information, there is a viable reason that it is difficult.