The use of yesterday (AYER) indicates the past tense.
Put yesterday (AYER) AT THE START OF THE SENTENSE,  and use the present tense of the verb.

For example;

Yesterday I went to the beach.
AYER, voy a la playa.

The use of tomorrow (MAÑANA) indicates the future tense.
Put tomorrow (MAÑANA) at the start of the sentence and use the present tense of the verb.

For example;

Tomorrow I will go to the beach.
MAÑANA, voy a la playa.

As in English there are subject pronouns i.e.

SINGULAR
I    -  yo
you-  tÚ
he, she, it  =  él, ella, usted.

PLURAL
us -   nosotros (nosotras)
you   - vosotros (vosotras)
them, they = ellos, ellas, ustedes

Normally these subject pronouns are only used for emphases and are put in front of the verb. As you will have noticed there are 6 parts in the conjunction of the verb and in Spanish each part  confirms which person or group or thing is doing the action, therefore dispensing with the personal pronoun.

More Learning Spanish…

• Present tense Spanish
• Tips for learning Spanish
• Spanish Reflexive verbs
• Spanish Prepositions ‘a’ and ‘de’
• Spanish Prepositions ‘por’ and ‘para’