Understanding __________different parts of speech forms the base of leaning grammar.

Answer: Option B

Option A: Five
This option is incorrect. There are more than five parts of speech in grammar.

Option B: Eight
This is the correct option. There are eight traditional parts of speech in English grammar: nouns, pronouns, verbs, adjectives, adverbs, prepositions, conjunctions, and interjections.

Option C: Six
This option is incorrect. English grammar is typically categorized into eight parts of speech, not six.

Option D: Seven
This option is incorrect. There are eight parts of speech, not seven, in English grammar.

In conclusion, the correct answer is Option B: Eight, as understanding these eight parts of speech forms the foundational knowledge of learning grammar in English.