tai + nominative

Introducing and identifying

• Tai mano brolis. — This is my brother.
• Tai mano draugės Rūta ir Eglė. — These are my friends Rūta and Eglė. Tai stays the same in the plural.
• Tai yra Vilnius. — This is Vilnius. Yra may be added, especially in slower, careful speech.
• Tai buvo geras filmas. — That was a good film. The past form buvo works the same way.

Questions and negation

• Kas tai? or Kas tai yra? — What is this? You will also hear Kas čia? (What is here?).
• Ar tai tavo telefonas? — Is this your phone? See Yes-No Questions With Ar.
• Tai ne mano knyga. — This is not my book. Ne goes right before the noun phrase, or you can say Tai nėra mano knyga.

Tai is not "it" for a known thing
Tai introduces something new or points at it. Once a noun is known, refer to it with jis or ji according to its gender: Kur mano knyga? Ji ant stalo. Tai can also refer to a whole idea: Tai gerai. (That is good.)

Tai and tas
Tas and ta (that) are demonstrative pronouns. They can accompany a noun or stand alone, and change their form: tas namas, ta knyga. Tie mano draugai can be a phrase meaning "those friends of mine"; with context and intonation, tie can also stand alone as the subject of a sentence. To say "These are my friends", use Tai mano draugai. See This and That.