Translation:The guard went to see what had happened.
Just an etymological question: how did the past tense forms of ir and ser become the same?
It probably has something to do with the similarity of the concepts. If you think about the English equivalents, "to be" gives "was/have been" and "to go" gives "went/have gone". Although there are differences, they can convey the same basic idea: "I have been to Scotland", "I went to Scotland", "I was in Scotland" and "I have gone to Scotland" all imply travel to Scotland in the past.
Of course, this would sound more convincing if the Portuguese "to be" verb involved was "estar" not "ser", but I found an interesting note about Spanish which may have some relevance to Portuguese as well:
(1) Why do ser and ir have the same form in the preterite? First, it is really the perfect conjugation of the Latin verb esse which came to be used for both ser and ir in Spanish. In Spoken Latin, the preposition in (Spanish en) used with a location signified "movement toward". Also, ser and estar did not have the same restrictions in Old Spanish that they do in Modern Spanish and consequently ser occurred at times where one might expect estar. Thus fuerunt in campum = "they were (moving/in route) toward the country", e. g., eventually came to mean "they went to the country", and this paradigm eventually became associated with the infinitive ir (to go).
They come from Latin. In ancient (attic) Greek they are pretty close in some conjugations. But in Portuguese, the verb ir comes com the Latin eo (to go), but in Portuguese it's irregular: vords beggining with F came from Latin verb sum (to be) - that envolved to verb ser in Portuguese. The words beggining with V came from the Latin verb vado (to go, walk).
why is it 'tinha' rather than 'tem'? the "I" form, as opposed to the "he" form?
Past tense (indicativo pretérito imperfeito) Eu and Ele/Ela are the same. Check http://www.conjuga-me.net/verbo-ter. On a another subject I haven't found a good website explaining the differences of the verb tenses between English and Portuguese and how they relate to each other and their differences. I would find that very interesting if any one knows of one. There isn't a one to one relationship between the two from what I have heard.
There is a stalled attempt to produce an open-content textbook (Wikibook) about Portuguese and this section, although a work in progress, could be what you are looking for: