What word is mostly use to start a sentence?

There's no single word that's "mostly" used to start a sentence. It depends heavily on the context and style of writing.

Here are some common words used to start sentences:

* Pronouns: I, You, He, She, It, We, They

* Articles: The, A, An

* Conjunctions: But, And, So, Yet, However, Therefore

* Adverbs: Then, Now, Eventually, Finally, Often

* Prepositions: On, In, At, With, By, For, To

* Numbers: One, Two, Three...

Some words are more common than others depending on the type of writing:

* Narrative: "He," "She," "The"

* Expository: "Therefore," "However," "Moreover"

* Descriptive: "The," "A," "On"

It's important to use a variety of sentence starters to make your writing more interesting and engaging.