Question: May one eat matza balls (kneidlach) or crackers before Pesach?
Answer: The Gemara Yerushalmi (Pesachim 10:1) teaches that one must not eat matza on erev Pesach. Rambam (Chametz Umatza 6:12) explains that this makes it clear that the matza one eats on seder night is eaten for the mitzva. The Rema (OC 471:2) writes that this prohibition applies all day though there is a machlokes as to whether this starts the preceding night or from amud hashachar¸ daybreak (See Magen Avraham 471:6, Chok Yaakov 471:7).
The Mishna Berura (471:12) adds that some have the minhag not to eat matza from Rosh Chodesh Nissan. R’ Moshe Feinstein (Igros Moshe OC 1:155) notes that some abstain for thirty days before Pesach as according to one view in the Gemara (Pesachim 6a) this is when our Pesach preparations begin.
Tosafos (Pesachim 99b) writes that this only applies to matza that one can use to fulfil the mitzva (See Biur Hagra OC 444:1). Thus, the Baer Heitev (OC 471:5) writes that one may eat crackers providing that there is no possibility that they could be fit for matza. Likewise, the Shulchan Aruch (OC 471:2) writes that one can eat egg matza on erev Pesach before sha’ah asiris, three halachic hours before Yom Tov. Therefore, R’ Moshe Feinstein writes that when erev Pesach falls on Shabbos, one can use egg matza for lechem mishna on Friday night and in the morning.
The Rema (OC 471:2) writes that one cannot eat matza that had been broken up and kneaded with wine and oil. Thus, one would not be able to eat matza brei, etc. on Erev Pesach. The Mishna Berura (444:8; 471:20; Shaar Hatziyun 444:1) explains that even if one does so, it is still considered to be matza. However, if one cooked the matza, to make kneidlach one would be able to eat it up until sha’ah asiris.
R’ Shmuel Wosner (Shevet HaLevi 8:117) writes that this prohibition includes cakes baked from matza meal (See Piskei Teshuvos 471:3). According to R’ Shlomo Zalman Auerbach (Halichos Shlomo, Pesach 8:4), one may eat food made with matza meal providing that it does not have the ‘form of bread’. R’ Ovadia Yosef (Yabia Omer OC 6:39) writes that the prohibition only applies to actual matza but cake made with matza meal can be eaten.
In conclusion, there are different customs as to when the prohibition to eat matza begins, though this only applies to actual matza. One may eat kneidlach and meat balls made with matza meal on erev Pesach. There are different customs as to whether one can eat biscuits and cakes made with matza meal on erev Pesach.