Demonstrativpronomen: этот und тот

Э́тот means "this" and тот means "that". Unlike their English counterparts, both change their endings to agree with the noun, like adjectives.

Э́тот по́езд идёт в Петербу́рг? - "Does this train go to St Petersburg?" Э́та пиццери́я хоро́шая. - "This pizzeria is good." Э́ти пирожки́ с капу́стой. - "These pies are filled with cabbage."

Это: "this is" vs "this"

The neuter э́то has a second job. Used on its own as the subject, it means "this is / that is / these are", and then it never changes:

• Э́то кни́га. - "This is a book." Э́та кни́га - "this book".
• Э́то мои́ друзья́. - "These are my friends." Э́ти друзья́ - "these friends".

So the question to ask is: am I pointing and naming (э́то), or describing a noun (э́тот / э́та / э́то / э́ти)? English speakers who hear "these are" often reach for Э́ти мои́ друзья́, but that is a phrase ("these friends of mine"), not a sentence.

тот: used less than English "that"

English says "that" a lot. Russian keeps тот mostly for contrast and distance, often in a pair with э́тот: Не э́тот авто́бус, а тот. - "Not this bus, that one." Where English uses "that" to refer back to something already mentioned, Russian usually says э́то or э́тот: Э́то был отли́чный фильм! - "That was a great film!" Pick тот when you are pointing at something further away or setting two things apart.