Webster's Dictionary (the standard for American English) contains more than 400,000 entries. That's a whole lot of words for writers to try and track, and it's one of the reasons grammar is so difficult to master. Knowing which word goes where is pretty much impossible -- unless you memorize everything about all 400,000 of those words (including the correct spelling). Know your parts of speech instead, and you'll have an easier time making perfect sentences that won't confuse your readers.
Technically, there are just two articles in the entire English language: a and the. Sometimes, a turns into an, but they're considered to be the same word (vowels like to confuse everyone). Articles are only used with nouns. A sentence using an article with no noun would look something like this: The blue wandered past. Somewhere inside your head, a voice ought to be screaming the blue what? That's how you know there's a missing noun.
Remember the classic English lesson? Nouns are persons, places and things. Sally and Herbert are both nouns; so are balloon and Luxembourg. You can't make a sentence without a noun. Articles and adjectives cannot possibly function without them, and you'd have a pretty tough time using verbs, too.
Nouns are a big part of speech; there are a lot of words that fall into this category. But this part of speech is not without its confusion. Jade is noun...but she isn't. She is a pronoun.
She, he, her and we are all pronouns. A pronoun is simply a word that replaces a noun, and it's absolutely a necessary part of speech for a certain writing trick that everyone uses. If you write a sentence like Emma stared between Roger and Henry, Emma's green eyes lit with an angry fire as Roger reached to take Emma's hand. How repetitive and ugly is that sentence? You absolutely need the word her to avoid this type of redundancy, so hold your pronouns close to you at night.
It's incredibly difficult to write descriptive sentences without conjunctions. Also known as linking words, the conjunctions include, and are not limited to, for, and, nor, but, or, yet and so. You can't even describe the holiday season without a conjunction -- unless you know another way to write red and green. Conjunctions link sentence parts together and link words within the sentence together. Robb wanted to go to the movies, but I wanted to visit the museum. The Monet was red and blue. In order to properly use conjunctions all the time, you've got to know how to use commas.
Verbs are action words. You need them to walk, run, think or even just sit. All derivatives of be and do are considered verbs, so even if something just is you're using a verb. Traditionally, verbs are used with nouns. You can write To run through the park, but it doesn't make much sense and there's no narrative. Tell me that Molly runs through the park, and I understand what the heck is going on. Even when you put the verb in the sentence first, it's still describing a noun: Coughing and sputtering, the car climbed up the hill. The verbs in that sentence are cough, sputter and climb, and every single one of them is used with the noun car. If you can't find the noun in your sentence but you can find verbs, you might have a grammar problem.
Like the name suggests, adverbs have something to do with verbs. In fact, they're completely dependent upon them. Whether you know it or not, you probably use a certain adverb a lot: it's very. Adverbs are used to modify verbs. A proper sentence containing a verb and an adverb would look something like this: She carefully wrote the blog post. Obviously, to write is the verb in this sentence. The adverb carefully modifies the verb by helping to describe it. How did she write? Carefully, that's how.
Almost all adverbs end in ly. An example of one that doesn't is always. You can't always something unless there's a verb involved - you can always love grammar blog posts, because love is a verb. But you can never always blue car, because that's totally nonsensical.
No writer could effectively write without adjectives. All colors are adjectives, as are most feelings. Adjectives are otherwise known as descriptive words, because that's the purpose they serve. They describe. I can tell you that The wheelbarrow was sitting in the yard. But if I tell you that The rusted, brown wheelbarrow was sitting in the yard you can picture it a whole lot better, right? Adjectives are the writer's best friend, so use them well.
You cannot have an adjective without a noun or a pronoun. I can tell you blue, worn and ugly, but if you don't know what's blue, worn and ugly then you aren't going to care.
The easy answer for prepositions is that they are words describing anywhere a mouse can go. They're so confusing, I wrote an entire post about prepositions in the past. They're simply used to link pronouns and nouns, usually to each other, and I ignore all rules of preposition usage. Prepositions include words like to, under, over, with, up, toward and between, to name just a few. If you're between a rock and a hard place, you're properly using your preposition to link nouns. Prepositions, like many other parts of speech, can appear pretty much anywhere in a sentence -- even at the very beginning or the very end. Usually, but not always, prepositions are immediately followed by a noun or a pronoun.
Truly the most under-appreciate part of speech, interjections are great fun to insert into any sentence -- and they can go anywhere. Really, words that are classified as interjections have been given this label because they don't belong anywhere else. Wow! Oh! Yikes! and oops are all interjections, and no you don't always have to use them with an exclamation point (I just like exclamation points). Using them very occasionally can draw attention to a specific moment in your writing: And oh! He'd hurt me terribly that day. Wow, he's really changed.
Parts of Speech
Knowing the basic parts of speech will help you craft strong, grammatically-correct sentences. However, many words can be multiple parts of speech -- it just depends on where you put them in the sentence. If the leather is soft and smooth, the word leather is a noun. If the leather purse is soft and smooth, the word leather becomes an adjective. It's usually easiest to identify nouns and verbs first, so once you figure out where these guys are at you can often sort out all the rest.