More than one translation may be perfectly acceptable (assign 1).

Some translations may be half right -- they sort of get the sense across. Do not reward these with a 0.5 or you will be training and rewarding half right answers. (Rather assign 0).  That is, any imperfection gets an assignation of 0 to the sentence. 

Thank you very much for the answer!  I have a follow-up question then. If the sentence was designed in such a way to get a particular vocabulary unit from the learner, but the learner paraphrased it and the sentence is still grammatically correct, is that a case for a 1 or 0?

E.g.: 'The grass is green' instead of the target 'The grass is EMERALD green'.

Hello Armine Gharibyan,

Notably, many translators take the sentence as the unit of translation since it serves as a complete propositional or idea unit. For the assessment of translated sentences, however, much depends on the theoretical beliefs of the interpreter. If the theory adopted underlies the semantic method, then a source text orientation is preferable. By contrast, theories suggesting invisibility of the translator opt for a target text orientation. Notably, in the case of sentence translation, you can use a back translation test and based on the degree of the existing overlap you can rate the targeted translated sentence.

Best of luck,

R. Biria

This is not an exercise in paraphrasing, but in translating. I feel that in the case of the translator giving "'The grass is EMERALD green", the student has perhaps unwittingly given more than was required. Think of what would happen if Google Translate suddenly gave that as a translation! You would mark it down somewhat. Perhaps 0.9. The back translate would give a somewhat different original sentence.

Thank you very much for all the answers. I'll take everything into consideration.

Dear if you want to evaluate the structure of sentences you can use syntax and measure those sentences within the frame of verb and noun.

1, 0, 0.5, 0.9 etc. will also depend on what you intend to do with these numbers, and what kind of maths you will believe applicable.