Grammarchecker.co will identify the most common spelling and grammar mistakes. But there are no guarantees that it will identify all the grammar mistakes in a submitted document. The reason for this is that grammarchecker.co checks each sentence for grammar mistakes. However, it does not take into account the tense in which certain sentences or paragraphs are written. For example if the document has present and past tense sentences in it, grammarchecker.co will not mark them as errors. Grammarchecker.co checks sentence grammar and spelling. It starts checking with the first capital letter and the full stop or question mark or exclamation mark. It assumes the words starting with the capital and ending at the end of sentence punctuation mark to be a complete sentence. It will check the spelling and grammar of this sentence. In case it finds any errors it will mark it and link it with the appropriate message to be displayed. After it has checked the first sentence, it will move on and repeat the process for the next sentence. It will continue checking in this manner till it reaches the end of the document. After which it will stop and display the document with the mistakes highlighted in different colors. Now as you will have noted it has done a sentence by sentence check; it has not checked the tense of the sentence or its length. Therefore if a sentence is too short or too long it won’t mark it as an error. Similarly if the document has sentences in any of the tenses (Present, Past, Future) it will just check the sentence and not the tense. If the sentence is correct it will not mark it.
The reason for grammarchecker.co performing a sentence by sentence check is that there are various types of documents that are written. Some documents might be written in the past tense, others in the present and yet others in the future tense. Also some documents may have a mix of tenses used in them. Grammarchecker.co does not check the context of the document as that frankly would be an impossible task to undertake.