Most words that are plurals require a letter 's' to be added...but not all do! This quiz will give you some practice in writing the plurals of nouns. By the way, adjectives don't have plurals: 'A blue pen'; 'Two blue pens'. Also, some nouns don't have a separate plural form.
It's easy to be sloppy with spelling ~ especially as most people will understand what you are trying to write. However, firstly it's bad English and secondly, it's not really fair on your readers to make them have to 'work out' what you are saying. Much better to write clear, standard English with no room for misinterpretation. Not only does good spelling make a difference; using the correct word (i.e. its meaning) is vital. Many misunderstandings can be avoided by learning how to use the English language properly.
See how you fare in this enjoyable 11-plus spelling quiz.