What is an “awkward sentence”?

From LitWiki

Wordy sentences are often awkward, but not all awkward sentences are wordy. But like wordy sentences, awkward sentences rarely withstand the oral test. If you stumble when reading your prose aloud, chances are the sentence needs to be revised.

Awkward sentences often result from unusual syntax, the omission or addition of a word, and/or unclear or complex thoughts. Here are some examples:

Awkward: Ed ate caviar on his birthday, which cost fifty dollars.
Revised: Ed ate fifty-dollar caviar on his birthday.

Here, the price of the caviar causes confusion. In the former sentence, the reader is unsure whether the birthday or the caviar costs fifty dollars. By changing the noun into an adjective, and placing the adjective closer to the noun it modifies, the sentence is made clearer and less awkward.

Grammatical errors often contribute to awkward sentences:

Awkward: Dentists and doctors receive similar training, the public views them with less respect.
Revised: While dentists and doctors receive similar training, the public views dentists with less respect.

The former sentence is awkward because the subordinate clause is not subordinate, causing a comma splice, and “them” is an ambiguous pronoun — does it rename “doctors” or “dentists”? By simply adding “while” at the beginning of the subordinate clause and changing “them” to the specific group, in this case “dentists,” the sentence becomes grammatically correct and understandable.

His mother dipping him made him near invulnerable in the river Styx. The idea of heroes has been around since humans’ earliest writings.

Often the arrangement of phrases and words can cause confusion. Also, make sure that when an adverb is used, an "-ly" is added when necessary. Therefore, the first awkward sentence can be revised:

Dipping him in the river Styx, his mother made him nearly invulnerable.

This second example presents a more difficult problem. A good writer must always have a clear idea of just what s/he wants the sentence to communicate. The most important part should be the subject, here “heroes.” Secondly, choose a strong verb. Since the topic here is heroes, the writer choose a passive construction. Next, conclude the sentence in the most succinct, lucid way possible by replacing the awkward and grammatically flawed phrase “has been around since mankind has earliest writings”:

Heroes have been the subjects of humanity’s earliest writings.

A bit of thought and proofreading will help you avoid awkward sentences.