What do they express?

Feelings, emotions that are connected or pertain to their selves. This may be the impact through their works as a writer or author, you'll get the emotion from the poem that they have wrote based on their point of view.

When do we usually use them? What are they called?

we use it mostly position between the subject and the main verb, or after the modal verb or first auxiliary verb or be as a main verb.

Explanation:

Hope it helps

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