Modal verbs such as: "to be", "to have", "can" "must" "want to", "could", "would" are not on the word list because, the 625 List is designed to provide two things:
1) Extremely frequent, useful words [Modal verbs definitely fall into this category]
2) Words that can be easily learned using pictures alone
The goal for these 625 Lists is to give users a way to learn a bunch of key vocabulary without needing to resort to translation. Once you progress to sentences and grammar, you should already have a solid vocabulary foundation, and can better focus on grammar and abstract vocabulary.
For more info on how to learn abstract words and phrases, have a look at these articles: