The word enjambment comes from the French verb enjamber, meaning to span, straddle or stride, literally to step seamlessly from one line of verse to the next. Looking at punctuation often seems a good way to spot it. If there’s punctuation at the end of the line, the line is end-stopped, i.e. you pause at the end of the line. If there’s no punctuation, then the line is enjambed (or run-on, an alternative term) because you carry on reading seamlessly over the line-break.
The first thing to bear in mind is that the punctuation you will see in a modern edition of a medieval poem has been put there by an editor rather than by the poem’s author. Chaucer and his scribes used a different system of punctuation, which most editors of student editions replace with modern punctuation. So you can’t rely just on the punctuation to indicate what a poet wanted you to do at the end of the line. Commenting on enjambment thus requires you to consider the relationship between lineation (the dividing up of a poem’s language into lines of verse) and syntax (the arrangement of words and phrases in a sentence).
Most Middle English poetry written before Chaucer (though not all, see here for an exception) matches up its units of meaning (i.e. its phrases and clauses) with its line endings, and enjambment is pretty rare. Sentences do go on for several lines at a time, but usually the end of a clause of phrase matches up with a line-break. The symmetry of form and meaning is mutually reinforcing. Line-breaks emphasise syntax and syntax emphasises line-breaks.
Chaucer’s verse often features a more lopsided arrangement, where clauses begin and end in the middle of a line. This lopsidedness makes you pay more attention, as the line-breaks don’t neatly correspond to the ends of phrases and clauses. There is a tension between the poetry’s regular patterns (of metre, line-length and rhyme) and the syntax which is fitted into those repeating patterns in an off-centre fashion. This, paradoxically, makes the verse seem effortlessly natural because the rhyme is not over-emphasised. The poetry is less obvious in its correspondences, more cognitively interesting. Having created this alternate style, Chaucer (and later writers who imitated him) can contrast passages of endstopped verse with sequences of enjambed lines for various purposes.
Here’s an example of enjambment from the beginning of Book V of Troilus, when Antenor is exchanged for Criseyde:
And right with that was Antenor y-come
Out of the Grekes ost, and every wight
Was of it glad, and seyde he was welcome,
And Troilus, al nere his herte light,
He peyned him with al his fulle might
Him to with-holde of wepinge at the leste,
And Antenor he kiste, and made feste.
Clauses straddle the line-breaks in lines 1–2 and 2–3. A description of Troilus’s concealing of his sorrow steps across lines 4 to 7. Remember that that enjambment works in combination with other poetic elements. Enjambment can be emphasised or reinforced by complexities, inversions or dislocations of word order (such as Was of it glad line 3, rather than the expected Was glad of it, thus emphasising the adjective, ironic because this is the return of Troy’s traitor). It can be reinforced by regularity in the metre, sweeping you along from line to line.
Many of the stanzas in Troilus have enjambment, so you need to have a good reason why you want to pick a particular example out. One way to talk about enjambment and endstopping is to argue that they function iconically, i.e. that they somehow resemble or imitate something which the words of the poem describe. There are lots of possibilities. The flowing of syntax over line-breaks might imitate or evoke the flowing on of (for example) argument, thought, movement, time. Endstopped lines might imitate or evoke certainty, stasis, regularity. The spatial move of dropping down from one line to the next, or moving across the space between stanzas, might evoke a pause, a hesitation, a lurch.
It’s important to remember that this is not a property of enjambment or endstopping in themselves, but needs to be triggered as a possibility by the content of the lines or the ingenuity of your close-reading. Sometimes the enjambment doesn’t evoke but rather contrasts with the meaning of the lines. In the stanza given above, lines 4 to 7, which flow on fluidly and expressively, in fact describe pain, disguise, effort, holding things in. So enjambment is not always straightforwardly imitative or iconic – your interpretation needs to be plausible both in terms of form and meaning.