(Mishnah): A man must say three things in his house shortly before dark [on Erev Shabbos] - "did you tithe? Did you make an Eiruv? Light Ner Shabbos!''

If it is Safek dark (i.e. Safek night), one may not tithe Vadai (definite) Tevel, immerse Kelim, or light Ner Shabbos;

[At this time] one may tithe Demai (Safek Tevel), make an Eiruv, and do Hatmanah (wrap food to keep it warm).

(Gemara - Beraisa - R. Shimon ben Gamliel): Halachos of Kodesh, Chata'os and Hechsheros are basic Halachos, and all of them, Amei ha'Aretz are trusted about them. (This is brought to show that our Mishnah is not because women are not trusted about these matters. Rather, it is to make people zealous to do these matters before Shabbos - SFEER MASHBI'ACH.)

Halachos of Kodesh - a Mishnah teaches that if he says 'I separated a Revi'is of Kodesh into it', he is believed.

Chata'os - a Mishnah teaches that all are believed about Chatas (i.e. Parah Adumah).

Hechsheros - a Mishnah teaches that an Am ha'Aretz is believed about all of them, to say that they are Tehorim (did not receive Hechsher Tum'ah).

Question: Should he not say [first what is forbidden mid'Oraisa on Shabbos, i.e.] light the lamp, did you tithe, did you make an Eruv?!

Answer (R. Chiya bar Aba): Because you are stringent with him about light (mid'Rabanan) Isurim, he himself is stringent about stringent (Torah) Isurim.

R. Chagai: R. Shmuel bar Rav Yitzchak used to command in his house "did you make an Eruv? Light the lamp!''

Question: Why did he not say "did you tithe?''

Answer: Everything that he ate, he bought in the market. (He bought Demai (R. SHLOMO SIRILIYO Demai 2:2), which one may tithe even when it is Safek dark. And if he bought from a Chaver, he need not tithe it at all! - KORBAN HA'EDAH)