Ein does not distinguish masculine from neuter by itself
In ein guter Plan and ein gutes Buch, the adjective makes the two patterns visibly different. This is a useful reason to learn complete phrases, rather than one adjective ending in isolation.
Das ist ein gutes Buch.
That is a good book.
Masculine accusative changes to -en
Compare subject ein guter Plan with object einen guten Plan. Feminine and neuter retain their adjective endings: eine gute Idee, ein gutes Buch.
Wir brauchen einen guten Plan.
We need a good plan.
Possessives and kein use this family of patterns
The contrasts also appear in mein guter Plan, meinen guten Plan and kein gutes Buch. Plural after a possessive or keine takes -en: meine guten Bücher.
Meine guten Bücher stehen hier.
My good books are here.
No-article plurals use -e in nominative and accusative
Compare gute Bücher with die guten Bücher or meine guten Bücher. Removing the determiner changes the adjective pattern. There is no plural article corresponding to singular ein.
Ich lese gute Bücher.
I read good books.
Uncounted amounts can also have no article
Use phrases such as warmes Wasser and frische Luft. Masculine accusative has -en, as in heißen Tee. These are nominative/accusative patterns; dative needs its own forms.
Ich trinke heißen Tee.
I am drinking hot tea.
Practise this pattern
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References
Original teaching prose and examples, informed by these grammar references. A2 placement is an editorial curriculum choice, not a CEFR certification.