Learning how to write a good thesis statement means learning
to cull potential duds. Some essential thesis sentence characteristics
are obvious or at least they should be obvious.

Unfortunately, when students are just learning to write, they
are thrilled just to get words on paper. It won't occur to them
to check to see if what they wrote is actually a statement.
If they won't look for the obvious things like a complete
sentence, they certainly won't look for subtle differences
between a good thesis and a poor one.
No matter how many examples of thesis sentences you give students,
they won't learn the craft of creating a thesis capable of guiding
their entire essay until they discover it for themselves.
The cause-and-effect relationship between a good thesis and a
good grade becomes clear to students only through trial
and error.
7 signs of thesis viability
There are seven signs of a potentially viable thesis. The four
signs near the top of the list are the most important and
quickest to learn. Students will need a significant amount of
writing practice before they see the importance of the later items.
1) Single-sentence format
Thesis statement is a synonym for thesis sentence.
Sentence fragments
like "causes of poor spelling" or "narrative poems"
cannot be used as a thesis statement because they are not complete
sentences.
By the same rule, two or more sentences or sentence fragments
won't cut it. The main idea must be one complete sentence, no
more, no less.

2) Declarative sentence
A good thesis must not only be a sentence, but it must also be
a declarative sentence.
Writers cannot use a question as their main idea and
expect to produce a decent piece of writing. A question straddles
an issue instead of taking a stand.
Writers need to take a stand in order to explore an idea thoroughly.
If need be, they can change their position after they gather evidence.
A sentence has to express an opinion even if it's
a timid opinion or it can't function as the main idea of
an essay.

3) Clear, precise terms
A good thesis sentence must be free of terms that could be interpreted
differently by different readers.
For example, one person might understand "lots of people"
to mean more than would fit in his living room. Another person
might understand "lots of people" to mean more than
half the world population.
Rather than be vague, writers should use specific terms in their
working thesis even at the
risk of guessing or exaggerating.
If writers have to do formal research to develop their essays,
the main words in the thesis sentence become search keywords.
A phrase like "all college-bound seniors" or "Democratic
voters in Mercer County" will be a better search term than
"lots of people."
After writers sift the evidence pertinent to their theses, they
can reword their working theses to fit the facts before they put
them into their papers.

4) Concise wording
Like a road sign "Franklin, next right," all the statement
has to do is keep the writer headed in the right direction. The
wording, however, does need to be specific. Concise and short
are not exact synonyms. "Franklin, turn wherever" will
not do.
Urge students to keep to fewer than 25 words
the fewer the better as long as those words are well
chosen.
The initial working thesis is just for the
student's personal use. Nobody else has to see
it, so it doesn't have to be fancy.
I find it convenient to remember the four essential characteristics
of a good thesis by the initialism CCDS:
Clear,
Concise, Declarative
Sentence.
Learn
three ways to make a good working thesis better.