Morphology
What to master
Morphology covers parts of speech and their grammatical categories. For nouns: gender, number, case, declension and indeclinable nouns. For adjectives: agreement, comparative and superlative forms. For numerals: compound and collective numerals and their government. For pronouns: categories and case forms. For verbs: aspect, transitivity, reflexivity, mood, tense, person and number; also participles and gerunds as verb forms. Adverbs, category-of-state words, prepositions, conjunctions and particles complete the system.
Traps
Homonymous forms are common: что can be a pronoun, conjunction or particle; как can be an adverb, conjunction or comparative particle. Compound numerals require care: пятьдесят, пятидесяти; collective numerals combine with masculine nouns and plural-only nouns (двое друзей, двое суток), not with feminine nouns. Indeclinable nouns keep their form but still have gender (пальто - neuter). Do not confuse a short adjective with a category-of-state word. Aspect pairs must be distinguished from tense forms.
Worked example
In «Он сказал, что придёт», что introduces a subordinate clause and is a conjunction; it is not the pronoun что. In «Что ты читаешь?», что is a pronoun. Rule of thumb: ask what the word does syntactically and whether it can be replaced by который.
Rule of thumb
Identify the part of speech from form, meaning and syntactic role together; one feature alone is unreliable.