An adult permanent resident is eligible to apply after at least two years as a PR; approval remains discretionary.
Sources: Singapore ICA — Becoming a Singapore citizen · Constitution of the Republic of Singapore — Citizenship
A person born in Singapore is a citizen by birth when at least one parent is a Singapore citizen, subject to the constitutional exceptions.
Sources: Constitution of the Republic of Singapore — Citizenship
A person born outside Singapore can acquire citizenship by descent through a Singapore citizen parent, subject to registration and additional connection rules where the parent is also a citizen by descent.
Sources: Constitution of the Republic of Singapore — Citizenship · Singapore ICA — Becoming a Singapore citizen
A foreign woman who marries a Singaporean man is eligible for a reduced two-year residence, instead of about ten.
Sources: Constitution of the Republic of Singapore — Citizenship