Sat - sitting
- sat is, first, the past tense; and second, the -ed ('past', or 'passive') participle. The former, the past tense, is the form to use when you say what you did yesterday - "I sat over there during the events", "She often sat sideways in her chair." The latter, the past participle, is the form to use with the auxiliary verb 'to have', "Have you ever sat in the cockpit?"
- sitting is the -ing ('present', or 'active') participle. What this means, in less technical language, is that you should use this form with the auxiliary verb 'to be' - "I am sitting here"; "You will be sitting in row AA." You can also use it as a participial adjective to describe your position: "Sitting here in the sunshine, I am enjoying my holiday", or "She brushed her hair sitting in front of the mirror."
This is a grammatical nicety that can cause difficulty to native students more than to non-native speakers. This is particularly true as the grammar of informal English, in the North of England at least, has different rules from those of formal English. The problems come because the local (colloquial) grammar allows "I was sat" - the use of the apparently -ed participle with the auxiliary verb 'to be'. (In its most extreme form, this can be "I was
sat sitting".) Never be tempted to say this in academic writing. You may find seated a helpful addition to your vocabulary if sat/sitting gives you trouble.