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It looks like she'll be buying a car soon.
Literal
[topic-は] soon car [object-を] buy-looks-like.
もうすぐ ('soon, before long') is the time-adverb pairing of もう ('already') and すぐ ('immediately') — 'already-soon,' i.e., the moment is approaching. 買いそうだ uses the ~そうだ of visual/conjectural evidence — 'looks like she'll buy.' Crucially this is NOT the hearsay ~そうだ (which would attach to the dictionary form 買うそうだ) — the difference is conjugation: stem + そうだ for visual evidence vs. plain form + そうだ for hearsay. Beginners often conflate these; the conjugation difference is the diagnostic.