Learn Russian: Это люди и деревья.

The singular form is человек (person). In Russian, "person" and "people" use completely different root words, just like in English! You will use человек for one person, and the irregular plural люди when talking about multiple people.

Since "people" and "trees" are plural, why does the sentence start with Это instead of the plural Эти?

When you are introducing or pointing things out using the structure "This is..." or "These are...", you always use the fixed, unchanging word Это. You would only use the plural Эти when it is attached directly to a noun, like in the phrase "these people" (эти люди).

Why does the plural of "tree" end in -ья (деревья)?

The singular word is дерево (tree). Some Russian words have special, irregular plural forms that end in a soft sign and я (-ья). You are learning about words like братья (brothers) and друзья (friends) in this lesson; деревья follows this exact same irregular pattern!

Does the stress change when going from the singular word for "tree" to the plural деревья?