[ introduces the building of the Tx2] with אם (“If you make an altar…”), implying that it’s an optional altar . However, Rashi mitzvah [ cites R’ Ishmael who says that while אם in the Torah usually refers to an optional action, this is one of three exceptions where אם really means כאשר (“when”) and the mitzvah is obligatory. Why then does the Torah describe building the altar with אם rather than כאשר? Tx7]
אם does express a type of condition: building an altar will only be obligatory if klall yisrael merits to enter the land of Israel. However, once klall yisrael enters the land, it will be obligatory.
The אם indicates that we need to do work before this obligation falls on us. We can’t get overly excited by the end goals that bring us closer to Hashem (i.e., building the altar, lending, first fruits) and skip the steps required to get there. Instead, we need to focus on the the hard work and preparation needed to make ourselves into people who can fulfill these mitzvot.
Building the altar really is obligatory, but the conditional “אם” tells us what our kavana should be. Hashem wants us to build it as though we weren’t obligated to, because we have a desire to do so (and would do it even if it were optional) not because “Hashem said so.” If we build the altar only because it’s obligatory, it’s not proper avodah.
The mitzvah of building the altar is mandatory, but אם describes a condition within that obligation. Chizkuni learns that the “stones” in the pasuk aren’t literal and encode some flexibility to build the altar out of slightly different material, like earth. However, if the Jews do build the altar specifically out of stones, then those stones cannot have been shaped by metal tools.