'Forest' is a singular countable noun in this sentence, so it needs to have an article (or another determiner) before it. So you can say 'I went to the forest today' , if it's a particular forest that everyone knows about, or if there is only one forest near your home and it's obvious that this is the one you are talking about. Otherwise, you can say say 'I went to a forest today' if there are various different forests near your home and you don't want to specify which one you went to. You can NEVER say *'I went to forest today'. This is not proper English. You always need to have some kind of determiner before a singular countable noun - either 'a' or 'the', or another word such as my/your/this/that/another. (NB The only time that the word 'forest' can be used in the singular without an article or determiner is when the word is being used in an uncountable sense. Uncountable nouns (e.g. sugar, water, snow, petrol) can be used without an article. So you could say 'The country is covered in forest', but this refers to the type of vegetation as opposed to a specific place to visit. This is a different meaning from 'a forest', as in your example above, which is countable.)

перед всеми исчисляемыми существительными нужен артикль, влючая forest