Elaine B. answered 06/02/19
Fun and Enthusiastic ESL and English Tutor
Flawed pronunciation can be a problem if the speaker can’t make accurate distinctions between a few critical vowel sounds (for example, the difference between want and won’t).
English speakers are generally aware that some consonants, like th, are difficult, so we understand when someone uses substitute sounds. A French speaker who says, “Zis is zee best ting I have heard!” would be easily understood. But a Brazilian speaker who says “I want do that” (when they are trying to say “I won’t do that.”) will have more problems. Vowels are the crucial sounds to master.
If you learned a British pronunciation, there may be a problem with how some of your vowels sound to American ears. For example, the deep “a” sound in the British pronunciation of words like “awful” might sound like a “o” to an American, and get confusing.
But if you’re fine on the important vowel sounds, then you probably don’t need to worry about your accent. Some of the other things you mentioned, like vocabulary and grammar, are important too. And speaking rhythm (emphasizing the key syllables in a sentence) is critical to comprehension.
There are lots of books about the American pronunciation, and one I particularly like (Mojsin, Mastering the American Accent) has a whole chapter devoted to helping native French speakers with their particular pronunciation issues. I just checked Amazon and there are equivalent guides to British English pronunciation as well, if you wanted to stay close to your schooling.
Hope this helps!