A green card is for people who live in the US. You may travel, but long trips can look like you moved away. That is called abandoning your residence.
Under 6 months: usually no problem coming back.
6 months to 1 year: you can usually come back, but CBP may ask questions about where you really live. It can also break continuous residence for citizenship.
More than 1 year without a re-entry permit: your green card no longer works to come back. You would need a returning resident (SB-1) visa from a US consulate, which is hard to get, or a new green card.
Short trips can also be a problem if you clearly live abroad and only visit the US.
Keep a US address, bank account and job or business.
File US tax returns as a resident.
Keep your driver's license and phone number.
Re-entry permit: for long trips
If you will be away more than 1 year (for example to care for a parent, or a work assignment), apply for a re-entry permit first.
File Form I-131 while you are in the US. The fee is $630.
Go to your biometrics appointment in the US. Plan your trip after it.
The permit is usually valid for up to 2 years. You can ask USCIS to send it to a US embassy abroad.
A re-entry permit helps you come back. It does not protect your citizenship clock.
The same rules apply. Also plan to be in the US to file your I-751 or I-829 on time. See Conditional green card: removing conditions with I-751.
Refugees with green cards
Travel with your own passport only after legal advice, or use a refugee travel document. See Refugees and asylees: proof of work and travel.
Leaving for 13 months "by accident" without a permit.
Leaving right after filing I-131 and missing biometrics.
Thinking a valid card means everything is fine. The time away matters, not the date on the card.