There is no such thing as a competent grammer checker, bar the one on the top of your neck. No computer program written so far or in the forseeable future can remotely approach the ability of a human being. Humans are hard-wired to check grammer: one of the very few things that set us apart from our close relatives in the genus Homo is the part of our brain that understands and creates grammer. This is unique: no other animal has it. Further, it exists and is functional even in completely untrained people.
If you lack confidence in your grammer, then training is the answer, not software. A little training - and I do mean a little - can turn any human into a competent grammerian. There may be better methods, but the best one that I am aware of is simply to write and have a competent teacher review your text and make appropriate corrections until you are writing perfectly. Most - probably all - people with "poor grammer" actually have excellent grammer, and are very consistent with it. Their problem is not at all that they have "poor grammer", it's simply that the grammer they learned as a young child is grammer from a language that is not quite the language they are "supposed" to speak. (such as English or French).
One does not need to teach such a person "how to do grammer", they already know this, one merely needs to teach them the slightly different rules of grammer for the language they wish to speak or write in. (Such as formal English as opposed to colloquial or reigonal English). So if you "can't do grammer" and "need a computer to help", do not despair! You don't need to do the grammatical equivalent of learning to drive a car, you only need to do the equivalent of learning to drive a Ford when you have grown up driving Volkswagens.
This inbuilt, hardwired ability we humans have to create and apply grammatical rules is not only fundamental to our ability to think, conceptualise, communicate and build civilisations, it is astonishingly robust. One of the most graphic illustrations of this is the rapid metamorphis of pigeons and creoles. When you throw together a group of people who do not have a common language (such as on a construction site with many foreign laborers), they rapidly develop a pigeon - a made-up and very rudimentary language, usually based on one or two or three of the languages that the people do speak but do not share, which has very little flexibility and is hopeless for expressing abstract concepts but gets them by in the short term.
However, pigeons never last very long. The adults will carry on speaking pigeon for as long as they remain in the language-poor environment, but their children do not. These children grow up without the benefit of a common language to use amongst themselves. The lucky ones speak (say) Italian or Malay in the home, but must make do with pigeon outside. The unlucky ones don't even have that much - for their parents themselves do not have a common language (he speaks English, she speaks Bantu, say) and even in the home the only language spoken is pigeon.
So what happens? The children invent a new language! Incredible, but true. A real language, complete with all the complex forms and rules that characterise English or Mandarin or Russian or Bantu, and equally capable of expressing fine shades of meaning and abstract concepts. It has been studied and documented many times. Children have an innate ability to absorb linguistic rules, to understand verbs and nouns and pronouns and all the tenses, and this ability is so much a part of the human being as hard-wired at birth that language-deprived children will invent their own new language, and independantly turn their parent's clumsy pigeon into a true creole, with the same sort of rules that English has.
One does not need to consciously understand these rules in order to be able to use them. I certainly don't! Cliptin does - he can cite the component parts of any given sentence and give them their proper names - but that formal schooling is not needed - as witness the fact that Cliptin and I have roughly equivalent ability to express ourselves correctly, despite the fact that he knows how he does it and I don't. In fact we all have this ability, every last one of us. A Cliptin, with his formal schoooling, can quickly adapt to a new and different grammer (or at least I think he would be able to do so), where I must rely on my ingrained exposure to the language that I grew up speaking (fairly formal and correct English) and would have to take longer practicing to speak a different language were I called on to master it.
But master it I would - for like all humans, I was born with the ability to do so. Computers do not.
