The time when you can file Form I-485, Application to Register Permanent Residence or Adjust Status, depends on your eligibility category. Whereas, the eligibility requirements for adjustment of status vary depending on the immigrant category you are applying under.
According to the Instructions for Application to Register Permanent Residence or Adjust Status, you may apply as the person who directly qualifies for an immigrant category (principal applicant) or, in some cases, as a family member of the principal applicant (derivative applicant).
Generally, if you are filing as a beneficiary of an immigrant visa petition, such as: Form I-130 — Petition for Alien Relative, Form I-140 — Immigrant Petition for Alien Worker, Form I-360 — Petition for Amerasian, Widow(er) or Special Immigrant, you may file an adjustment application only after USCIS has approved your petition and an immigrant visa number is immediately available. However, there are some immigrant categories that allow you to file Form I-485 before USCIS approves your petition (this is known as “concurrent filing”), provided that approval of the petition would make a visa number immediately available and you meet all other filing requirements.
As for derivative applicants, USCIS cannot approve your Form I-485 until the principal applicant has been granted the lawful permanent resident status (with the exception of U nonimmigrants, asylees, and refugees).
If you are currently the spouse or child (unmarried and under 21 years of age) of a principal applicant, you may file Form I-485 if an immigrant visa is immediately available to you and you meet all the filing requirements. You may file at the same time or after the principal applicant filed his/her Form I-485.
If you are filing based on Form I-129F, Petition for Alien Fiancé(e), you can file Form I-485 once you entered the United States on a K-1 visa and married the US citizen who sponsored your I-129F petition (within 90 days of your entry). Also, you can file Form I-485 once you entered the United States as a K-2 child and your K-1 parent is also eligible to file Form I-485.
Therefore, in most cases you have to prepare and file other related forms before filing Form I-485.