Hi Hafsa, It's unclear what you're asking in your question, but if you wanted to know why "a" is used and not "an" as you might expect, it is because of the sound of the word "useful". Because "useful" has a "yoo" sound at the beginning, we use "a". This is the same for other words like "university" or "unicorn". However, words which have an "uh" sound like "umbrella" or "uncle" would use "an" - for example, "an umbrella", "an uncle". In general, when words start with vowels (a, e, i, o, u) we would use "an", however, this is down to the sound of the word following it, not the letter. That is why you will occasionally see people write either "an historian" or "a historian" depending on how they pronounce the word "historian". Hope that makes sense!

The rule for "a" or "an" is a phonetic rule, not an orthographic rule. The rule is that the indefinite article "a" is used before a singular countable noun and becomes "an" when the following word begins with a vowel sound. an apple a red apple a banana an old banana "a" become "an" when the following word begins with a silent "h." an hour an hourly wage a hat [not pronounced with a silent "h"] a hotel / an 'otel [depends on the age and region of the speaker] "a" is used before words that start with "u" and "eu" when they are pronounced with an initial "y." a university [pronounced "yuniversity"] a universal truth [pronounced "yuniversal"] an ugly dog [not pronounced "yugly dog"] a European language [pronounced yuropean] More information: https://grammar.collinsdictionary.com/easy-learning/the-indefinite-article [excerpt] They joined a historical society. They joined an historical society. (old-fashioned English) They were staying at a hotel. They were staying at an hotel. (old-fashioned English)

Hello! We would use "a" in "a useful invention" here because we are not speaking about a specific invention, For example: "I wish that I could come up with a useful invention like a special machine to help people, that are paralyzed, to walk again." We would use "the" - "the useful invention" if we are speaking about a specific invention. "The scientist patented the useful invention before anyone could copy it." I hope that helps!