Methinks is an old way of saying 'it seems to me', which learner books often gloss as 'I think'. That gloss is fair for how people use it now — as a joke, or as a bit of Shakespeare. The messy truth is kinder to remember: it is not I plus think. In Old English the verb was þyncan, 'to seem', and me was 'to me', like the me in show me. So the whole thing meant 'it seems to me'. English had another verb, þencan, which became our ordinary think; the two later looked the same, which is why the line 'I think' feels right and is still a little wrong. You will meet methinks in plays and in teasing: someone putting on an old-fashioned voice. Nobody says it in a shop. There is a past form, methought, 'it seemed to me'. Though actually, if a student writes methinks in a school essay, the teacher will think they are being cute, not careful.