We use yesterday to refer to the day before today. We we say yesterday we aren’t talking about at any specific time.

For example: I walked to work yesterday.
Similar expressions: Yesterday morning, Yesterday evening, Yesterday afternoon

This expression is similar to yesterday; can be used with a lot of time references, that is to say, to explain the most recent time.

For example: Guillermo travelled to Cancun last month.

Similar expressions: Last time, Last week, Last month, Last year

These expressions show a period of time; the phrase is in singular.

For example: Kenneth and his wife had a baby one week ago.

Similar expressions: a month ago, one year ago, an hour ago, etc.

These expressions are plural and show a period of time. They are used before ago.

For example: I took my English exam 3 weeks ago

Similar expressions: a month ago, one year ago, an hour ago, etc.

This time expression means the same that: two days ago.

For example: I was in Amy’s house the day before yesterday.

This expression is similar to the expression the day before yesterday.

For example: I was in Colima.

Similar expressions:: The day before last, the year before last, the season before last, the weekend before last, etc.