The paper discusses the various meanings of the words būt and būti ‘to be’ in Latvian and Lithuanian and their acquisition. The aim of the research is to discover in what meanings these words are used by the learners of the second Baltic language, i.e., the learners whose mother tongue is a Baltic language and who are learning the other Baltic language – Latvian for Lithuanians and Lithuanian for Latvians. Both languages are closely related, and the meanings are similar. However, their grouping in each linguistics tradition is different. Therefore, this paper attempts to group meanings from both languages into a single system that could be used to compare usage tendencies. Such a system was made based on the Latvian grouping tradition but is not concise. The meanings in which būt/būti is used may depend on the topics of the texts. There seems to be an overuse of the location meaning in texts written by learners of Latvian; learners of Lithuanian seem to be using the verb in a larger variety of meanings. Perfect times are also more present in texts written by learners of Lithuanian. It seems to hint at a systemic difference between Baltic languages: Latvian tends to use perfect times in situations when Lithuanian does not.