There are a number of obvious and often talked about things that inexperienced writers do that out them. The most commonly discussed one is called by various names: telling, exposition, summary narration, sometimes excess backstory or too much description. This badger deserves its own post because it's a complicated thing that takes many writers a while to understand even when they know about it. More about that another time.
The other things I notice as an editor are more subtle, and also much easier to fix. Here are a few of them.
The use of the word 'then'
New writers often use 'then' as a segue for the actions of their characters.
'He loaded his bags into the car. He made sure the tires were full of air. He then got in his car and drove away.'
This is a perfectly grammatical set of sentences, but I'd be willing to bet if this use of 'then' is in one place, the entire book will be speckled with 'he then' sentences. The thing about 'then' is that it's an adverb. It obviously relates to the time of things. As a general rule you want to avoid overuse of adverbs, But more specifically, the time sequence of a series of events should be clear from the writing. If it is, you don't need 'then.' It's redundant clutter. If it's not, you need to rework your sentences. Adding 'then' is a weak way to resolve the issue.
A rare 'just then' or 'then' used in dialog can work fine. But using 'he then' the way you would 'and' is distracting and unnecessary.
Also, 'he then' is for some reason often used by novice writers instead of 'then he' which works a lot better as a time sequencer if you need one. (Though only occasionally, not left and right.)
The phrase 'once again', like 'he then,' doesn't add any additional meaning. It seems intended to avoid repeating 'again.' It doesn't. It only adds another word that isn't necessary. What can you use instead of 'once again'? For example:
'He studied the lock for a moment and this time he turned the key upside down before attempting to insert it into the slot.'
'After several minutes, he picked up the hammer and began beating the doorknob with it, if anything, even harder than before.'
In other words, write about what happened. Add something to the story with this action. Otherwise, why have the character repeat it?
Sentences must be parallel. What does this mean? It means that when you use conjunctions, you have to join constructions together that are similar. Here's an example of a parallel sentence:
'She doesn't like washing dishes or doing housework.
You can see that the two sections of the sentence joined together by the conjunction have a similar structure.
Here is the same sentence written without the parallel structure:
'She doesn't like to wash dishes or doing housework.'
Sentences in which the subject does a secret switch
Here's something I see I fair bit of:
'John picked up the ax and headed for the door. Swinging the ax back, Amanda stepped in front of him.'
Now the subject of the second sentence begins as John, but then a new subject is introduced. The first part of the sentence doesn't have a stated subject so the assumed subject of this sentence would be Amanda as she's the only stated subject. But we know that it's John because, well, he's the person with the ax.
This is an example of the dreaded dangling participle. 'Swinging' is the participle here. It's dangling because the subject to which it refers has stood it up.
How to fix this:
'John picked up the ax and headed for the door. As he swung the ax back, Amanda stepped in front of him.'
If you're going to change subject mid-sentence, you have to clearly state who the subject of the first part of the sentence is.
Sticking unrelated actions together with 'and,' 'but,' or a semicolon
'He walked to the door and there was a boy standing across the street.'
These two sentences have been stuck together with 'and.'
'And' is only a word. It's not magic glue. It doesn't create a relationship between two unrelated sentences. Sentences connected by 'and' should be related by subject, or joint activity.
'He leaned in and she kissed him'
You see that one of these is the result of the other. They're connected in time. They depict an interaction.
You can do the same thing with clauses.
'He walked to the door and saw a boy standing across the street.'
Now there's a relationship between these two things. The relationship is the 'he' subject is the one doing both the walking and the seeing. He's doing them in relation to each other. You assume that he sees the boy through the door.
This would be acceptable, and possibly even better:
'He walked to the door and looked out. There was a boy standing across the street.'
Some writers will try to splice these two sentences together with a comma. It looks better than 'and' stuck in there willy nilly, but it's still wrong. You need a full stop or the sentence needs to be altered. You can't stick two complete sentences together with a comma. (Unless they're part of a series. Like this: 'He walked to the door, the dog ran up to him, the policeman turned on the siren, and - boom - he passed out.)
Well, you can, but it will set you up for a visit from the grammar police.