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Terminal: run. Derivation Graphs: (left-to-right) compact / full; (top-down) full / compact.

Variants: Llega un tren / Llegan dos trenes / Dos trenes llegan / Llegaron dos trenes / Dos trenes llegaron
Ungrammatical: *Llegó dos trenes / *Dos trenes llegó

Unaccusative verb (llegar) (cf. English arrive here):

llega un tren     llegan dos trenes

arrive.SG.PRES a train.SG.M     arrive.PL.PRES two train.PL.M

un tren llega     dos trenes llegan

a train.SG.M arrive.SG.PRES     two train.PL.M arrive.PL.PRES

Help: blue = parse inside. Abbr.: WS: Workspace; Initial WS: initial heads for Merge after LEX lookup.

WS n, n = 1,2,... for intermediate stages after Initial WS. Final WS: when WS items converge on a single SO.
Bracketing: {...} (Merge). {...} (FormSet). At Spellout, {...} indicates free word order. [word] indicates word is optionally present.
Head Hd_{selection/features}, e.g. p_θ = predicate head p that seeks a θ-relevant (θR) IA from the WS. p_θ is a θ-seeker (θS).
v_see:θ:pst = head v has complement see, a θR EA from the WS and TNS. v is also a θS.
INFL seeks θR from its complement (not the WS).
Complementizer C comes in different flavors wrt. Force, .e.g. declarative C, interrogative C_Q and relative clause head C_rel.
Features, e.g. TNS pres/pst or DET the/a (shorthand for a bundle of semantic features).
Note: φ-features, e.g. 1sg, appear only at Spellout. Some words do not project, turn into features, e.g. determiners.

Words: un tren llega

Initial WS 1: llegar_θ v_llegar:_pres INFL_v:_3sg tren_un

WS 1: {llegar_θ, tren_un} v_llegar:_pres INFL_v:_3sg

WS 2: {v_llegar:_pres, {llegar_θ, tren_un}} INFL_v:_3sg

WS 3: {INFL_v:_3sg, {v_llegar:_pres, {llegar_θ, tren_un}}}

Final WS: {C, {INFL_v:_3sg, {v_llegar:_pres, {llegar_θ, tren_un}}}}

INT/EXT: {C, {INFL_v:_3sg, {v_llegar:_pres, {llegar_θ, tren_un}}}}

EXT: un tren 3sg-m pres llegar

Spellout: un tren llega

Parse found: un tren 3sg-m pres llegar

EXT: 3sg-m pres llegar un tren

Spellout: llega un tren

Blocked: inconsistent with input!

llega	un	tren		llegan	dos	trenes
arrive.SG.PRES	a	train.SG.M		arrive.PL.PRES	two	train.PL.M