莫大 (mò dà) is a formal, literary adjective meaning 'greatest, utmost' — it always modifies a noun via 的 and is used in set phrases like 莫大的荣幸 (the greatest honor). Unlike 太 (tài), which is an adverb for 'too' or 'very' in everyday speech, 莫大 is a fixed, written expression. 重大 (zhòng dà) means 'major, significant' (e.g., 重大问题), while 莫大 emphasizes the superlative degree of an abstract quality.