In Elizabeth Barrett Browning’s poem ‘If Thou Must Love Me’ a few literary devices have been employed by the poet.

Archaism:

The poet has used archaic language, i.e., the old-world words to bring in a sense of royalty and grandeur.

If Thou must love me, let it be for nought

In the above line ‘thou’ for you and ‘nought’ for nothing are used to create the archaic feel. ‘Thy’ ‘thou’, ‘thine’, ‘may’st’ — all contribute to archaism.

Metaphor:

There is a metaphor in the line “A creature might forget to weep” where the speaker compares herself to a creature.

Assonance and Consonance:

In some places vowels sounds are repeated in the poem. It is called assonance.

If thou must love me

Thy comfort long…

Again, in some places, the consonant sounds are repeated. This is called consonance.

For these things in themselves, Beloved, may

I love her for her smile — her look — her way

Though, these cannot be considered as alliterations, these repetitions are indeed deliberate and add to the poetic devices of the poem.