The sentence "Always speak a truth" is incorrect because the word "truth" is an uncountable noun and does not take an indefinite article "a". The correct sentence would be "Always speak the truth."
always: This is correct as it indicates frequency and doesn't affect the grammar of the sentence.
speak: This is correct; it is the appropriate verb in the sentence.
a: This is the error. The article "a" is incorrect before "truth," which is uncountable.
truth: This is correct as a noun, but it doesn't take the article "a" in this context.
\boxed{\text{a}}