There are regular rules for how to stress a word in Spanish...If a word ends in a vowel, or with the letter n or s you stress the second-to-last syllable (COmen). When a word ends with any other consonant you stress the last syllable (entenDER). If a word does not follow the above rules, you place an accent on the vowel of the accentd syllable (rePÚBlica). To make it easier to learn, write the rules for how to stress the words, and as you learn each word, pay attention to the syllable that is stressed, and emphasize it slightly when you speak. It will eventually become easier.
You have probably found your answer as it's been two years, but I just wanted to add for the sake of others who might stumble upon this discussion that with many very common words in Spanish (está, sí, tú, él), the accent mark is not about indicating which syllable should be stressed when you speak the word, but instead is indicating the meaning the word. There are many common words with similar spellings-- one meaning will carry an accent and the other will not. So, when you see el you know it means the, but when you see él you know it means he. Also, with questions, interrogatory words (donde, cuando, cual, que) will carry an accent mark: "¿Cuándo llega Julia?" But, cuando won't carry an accent when it's used in a regular declarative sentence. "Comeremos cuanda llegue Julia." This article might be helpful: https://blog.lingoda.com/en/all-about-spanish-accent-marks-and-word-stress/
Mucho means a lot and muy means very. You shouldn't always directly translate everything to English, but in this cause even in English it wouldn't make sense, the boy is much happy? You would say the boy is very happy.
Also, learn to catch cognates in Spanish. Muy looks like very and Mucho looks like much or a lot in English
In Spanish, you aren't lucky; you have luck (tener suerte) in the same way that you have fear, have heat, have cold, etc.
If you meant the expression "happy-go-lucky", according to WordReference.com that's "despreocupado" (carefree, unworried). Not quite the same as being happy, although I can see the connection.
Feliz means happy in Spanish. In Spanish, when you are referring to more than one person or something plural the adjective has to be made plural too by adding an s. For instance, they are pretty: Ellos son bonitos.
Now, in this case: What if you wanted to say they are happy? This is where Felices (not felice) comes in. There is more than one way to say happy in Spanish but let's stick with feliz. They are happy: Ellos están felices. You added an s because you are referring to something plural. The reason why z turns to es is something Im not so sure of but in my opinion it would be even weird to me if I wrote felizs.