今天 / 明天 / 昨天 / 现在【jīntiān / míngtiān / zuótiān / xiànzài】

A time expression goes before the verb. It can sit after the subject or in front of the whole sentence, but never at the end.

wǒ qù我去Běi jīng北京míng tiān明天 is wrong. English routinely puts the time last, and that habit is the thing to unlearn.

Which position

After the subject is the default. Fronting the time puts weight on it – useful when contrasting one day with another.

Time before place

When both appear, time comes first.

There is no tense

The verb does not change. 我去 covers "I go", "I went" and "I'll go" – the time word does the work, and once it is there, nothing else is needed.

Long to short

A full time expression runs from the largest unit down: 明天下午三点.