A word and a sentence have more in common than you think. To such an extent that it’s far easier to tell how are they the same than how are they different.
A phrase first. A phrase is, in some sense, a big word. “A word made of several words”. Why? Because it does exactly what words do: it encodes an object of thought (has semantic meaning), can join with other words and phrases (has nodes).
For example, “red” and “coloured like a tomato” are no different. Same meaning, same nodes (blood can be either of the two).
A delicacy for thought: say, all words in language are written with no spaces (like in Early Latin). How do you tell a word and a phrase apart? Say “burstintoflames” – is it a word or a phrase? Means the same as “inflame”. Right, it’s made of individual “pieces”, but so are words “werewolf” (wer + wolf), “overpower” (over+power), “nevertheless” (never+the+less).
So, the difference between a phrase and a word is far from obvious.
Here is an article about the “word” notion problem in Japanese language – enjoy.
A sentence is a big phrase or a huge word. It is different from phrase only in absence of nodes: a sentence is the supreme form of word clustering, and does not subordinate to anything. Other than that, it’s no different from a phrase. It encodes a single object of thought. One could say that sentence is a nominative – a word that does not need to be part of a phrase.
A delicacy for thought: again, how is “themoonisshininginthedarkskyfullofstart” is not a word? After all, it means “night”…
Q: A sentence “I walk across the street” clearly encodes several objects of thought: me, street etc. Isn’t it?
The same I could say about any word. “Cat”, for example, encodes lots of objects: whiskers, fur, eyes, feline behaviour. All objects are stacked into each other, and simplicity or complexity of any object is conditional and depends on abstraction.
It might feel that an average object of a single word is somewhat different from an object encoded by a sentence. Say, a hook:
And “A boy is fishing in a pond.”:
But no, sentence object is not intrinsically more complicated. We could easily call the second picture “fishing”. And it works with any image. You could try it yourself: just google images and try naming them in a single word. It works.
“Guys running on enclosed ground and play football.” It’s “stadium”. “Someone sitting in bushes with a gun and sneaks on a fox.” It’s just “hunting”.
Okay, we have to miss some detail to call complex objects in a single word (the same we do when we call a cat “cat” instead of describing its features). But on the other hand, a single-worded description is often more powerful and profound.
And speaking of complexity. What do you picture in your head when you hear the word “love”? How would you paint it? Describe it? It took Shakespeare a book to describe it. So perhaps, “single-worded” objects are even more complex than those encoded by wordy sentences.
Q: Why can’t sentences be subordinate?
Sentences can join into paragraphs or stories, even require each other to be semantically meaningful, but their connection is based only on meaning. Linguistically a sentence is a complete unit that is capable of bringing a thought of a complete object in one’s head.
Oh, almost forgot, a word can also be “multi-word”. “Jack-in-the-box”, for example.
As you can see, sentences and words are similar just in everything. And differences… Well… words are written without spaces. That would be all, pretty much…