The past tense in Croatian, called the perfekt, is built by combining the present tense of biti (to be) with a past participle. For example, "I worked" becomes radio sam when a man says it and radila sam when a woman says it: jučer sam radio cijeli dan and jučer sam radila cijeli dan both translate as "Yesterday I worked all day." The participle matches the gender of the speaker, so "I was" is bio sam in the masculine and bila sam in the feminine, as in bio sam u Zagrebu and bila sam kod kuće. In the plural, the form is bili smo, seen in bili smo na moru (We were at the seaside). Other common past participles follow this same gender pattern: išao and išla from ići (to go), as in išao sam pješice and išla sam autobusom; došao and došla from doći (to come), as in došao sam kasno and došla sam ranije; and irregular masculine forms like jeo (ate) from jesti, pio (drank) from piti, vidio (saw) from vidjeti, and rekao (told) from reći. With a second-person subject, you say radio si (Radio si previše — You worked too much), and for third-person plural, radili su (Radili su do kasno — They worked until late).
To negate a past-tense sentence, keep the participle but replace sam, si, je and so on with the negative forms nisam, nisi, nije. For example, nisam radio jučer means "I did not work yesterday," and nisam išao na posao means "I did not go to work." This pattern is consistent regardless of which verb the participle comes from, which makes it easy to negate any past-tense clause once you have the affirmative version in mind.
The future tense (futur I) uses a different construction: the infinitive of the main verb is followed by a short form of htjeti — ću, ćeš, će, ćemo, ćete, će. So radit ću sutra means "I will work tomorrow," ići ću u kino means "I will go to the cinema," and doći ćemo u osam means "We will come at eight." To make the future negative, the helping verb becomes neću, nećeš, neće and so on while the infinitive stays in place: neću doći means "I will not come," and neću moći doći sutra means "I will not be able to come tomorrow."