The Mishnah states the opinion of Rabbi Meir (and we rule like him) that for a condition to be valid it must be Koful.
Tosfos in 49b D"H Devarim She'Belev and Kesubos 97a D"H Zavin, establish a three level rule for Devarim She'Belev and conditions. The middle level being where one's intentions are known to some extent and a Guilui Daas is sufficient without the full laws of Tenaim.
The question arises according to Tosfos, why did the Torah choose to teach us the laws of tenaim at the parsha of Bnei God and Bnei Reuven. Since Moshe Rabeinu stated his intentions in detail prior to making the Tenai to Elazar, Yehoshua and the elders, a Guilui Daas would have been enough and there was no need at all for a valid Tenai ?!
Tosfos does not write that whenever one has made his intentions known that we do not need Hilchos Tena'im. Tosfos writes that there are situations in which Giluy Da'as is enough, such as the case of "Al Menas l'Meisak l'Eretz Yisrael," since we have an Anan Sehadi there, and therefore Giluy Da'as is enough. However, the Anan Sehadi cannot be built upon the Giluy Da'as alone, for, if so, we would never need Tena'im. Tosfos in Kesuvos adds another point that it must be the Derech to make such conditions. Therefore, he concludes that it only applies to one who sold his Karka in order to go to Eretz Yisrael, and not to one who sold his Metaltelin. Although his intentions are clear and he was Megaleh Da'as, still if it is not standard we need Tena'im.
D. Zupnik