Syntax: "Just when I believed I couldn't ever want for more" is a complex sentence starting with an adverbial clause of time ('Just when I believed...'). The main clause is implied but interrupted by the next line. 'Want for more' uses the preposition 'for' to indicate the object of desire.
Morphology: "Couldn't" is a contraction of 'could not', showing negation in the past. "Ever" is an adverb of frequency used for emphasis in a negative context. "Believed" is the past simple tense of the verb 'believe', indicating a completed past action.

Syntax: This is a compound sentence. 'I saw you smile' is the main independent clause. 'And my mind could not erase the beauty of your face' is a second independent clause joined by the coordinating conjunction 'and'. 'The beauty of your face' is the direct object of the verb 'erase'.
Morphology: "Saw" is the past simple of 'see'. "Smile" is a bare infinitive verb following the perception verb 'saw'. "Could not" uses the modal 'could' for past ability, negated. "Erase" is a base form verb following the modal 'could'.

Syntax: "Won't you let me shelter you" is an interrogative sentence in form but functions as an imperative request or plea. It starts with the negative question form 'Won't you' to make a polite suggestion or invitation.
Morphology: "Won't" is a contraction of 'will not'. "Let" is a causative verb in its base form. "Shelter" is used as a verb here (to provide shelter).

Syntax: Imperative sentences ('Hold on to the nights', 'Hold on to the memories') are used for direct pleas. This is followed by a complex sentence expressing a wish: 'I wish that I could give you something more that I could be yours.' The clause 'that I could be yours' explains 'something more'.
Morphology: "Hold on" is a phrasal verb in the imperative mood. "Wish" is a present tense verb introducing a hypothetical desire. "Could give" and "could be" use the modal 'could' for hypothetical ability in the present/future.

Syntax: "How do we explain" is a wh-question in the present simple. The object is the noun clause 'something that took us by surprise'. The following lines are noun phrases ('Promises in vain', 'Love that is real but in disguise') that likely elaborate on the 'something'.
Morphology: "Do explain" uses the auxiliary 'do' for question formation in the present simple. "Took" is the past simple of 'take'. "Is" is the present simple of 'be'. "In disguise" is a prepositional phrase where 'disguise' is a noun.

Syntax: A complex sentence. The main clause is 'I think that I've been true to everybody else but me'. The conjunction 'And' introduces a second independent clause: 'the way I feel about you makes my heart long to be free'. 'The way I feel about you' is a noun clause acting as the subject of 'makes'.
Morphology: "I've been" is the present perfect tense of 'be', indicating a state from the past continuing to the present. "Makes" is the present simple, third person singular. "Long" is a verb here, meaning 'to have a strong desire'. "To be" is the infinitive form.