I'm an undergrad teaching assistant of a physic BSc student's lab. My job is reviewing the (usually 8-10 pages long) reports the students make, arriving in electronic format. During the review, I add short comments (1-2 sentences) to the document: things like "that's false because this and that", "this would've been better such and such", "you also could've check that and that", etc. The students then get their commented report back graded by the professor. Related to the course, I never meet the students in person.
The main purpose of these comments, among letting students know what they did wrong, is to help them improve their experimental skills. Observing and documenting everything reasonable, interpreting the results, thinking further, being consistent, etc. As such, there're usually a lot of comments on every student's report - usually 3-5/page - even on A-graded ones.
My concern is it's kind of hard to write even constructive comments of this kind without the possibility of sounding somewhat cantankerous. The relatively lot of "that"s bad, that's wrong/missing/false/etc" comments I make on small, but not unimportant errors set up a negative tone as is, and in this atmosphere even writing something like "that wasn't strictly in the task, but it's interesting to think about this and that" may come down as negative. I try to add as much positive comments as I can - but the simple "Good!", "Clever!", "I like the approach" kind of notes feel falsely in large amounts, especially when the student didn't really do anything outstanding, only did what he was told. Expanding them with something like "That's good work. You could also check that and that" also turns into education.
I'd like to avoid coming down too testy: it would kill the purpose, making students dismiss my comments as "nah, that guy finds error in everything". I want the students to feel that I actually want to help them, and I'm on their side. The professor - of course - told them the purpose of the reviews, but I know that when I did this same lab course (a year ago), there were still (silent) words like that about our teaching assistants.
How could I make my coments more encouraging, without sounding false?