Some nouns in Russian can only be used with the singular form. Among them can be found nouns that define
Полезные ископаемые (minerals):
золото (gold), нефть (oil), уголь (coal), железо (irongt;), серебро (silver), медь (copper), etc.
Строительные материалы (building materials):
черепица (tile), песок (sand), тёс (boards for roofing), цемент (cement), etc.
Химические элементы, их соединения, лекарства (chemical elements and their compounds, drugs):
кислород (oxygen), хлор (chlorine), азот (nitrogen), йод (iodine), аспирин (aspirin), etc.
Сельскохозяйственные культуры, растения, плоды обычно мелкие по размеру (crops, plants, fruits usually of small size):
земляника (strawberries), черника (blueberries), малина (raspberries), просо (millet), овёс (oats), пшеница (wheat), картофель (potatoe), лён (flax), морковь (carrot), рожь (rye), etc.
Пищевые продукты (food):
осетрина (sturgeon), баранина (lamb), говядина (beef), сметана (sour cream), мука (flour), сахар (sugar), вино (wine), мёд (honey), молоко (milk), etc.
И бытовые предметы (household items):
мыло (soap), шерсть (wool), etc.
In this group we can find nouns that can be measured, but can't be counted and can't be divided into separate units of quantity. For that reason, they can't be combined with quantitative numerals, except when these compounds act as “the appearance to a volume of”:
один стакан молока (a glass of milk), пять пачек сахара (five packets of sugar), etc.
The measure of a mass, volume, weight, expressed in terms of combinations of the words it designes:
Measure of quantity (kilogram, meter, litre, etc.):
литр молока (a litre of milk), тонна серебра (a ton of silver), баррель нефти (a barrel of oil), etc.
Indefinite quantity (piece, crumb, scrap, etc.):
кусок свинины (a piece of pork), капля воды (a drop of water), крошка хлеба (crumb of bread), клочок бумаги (a piece of paper), etc.
The number of something as existing independently of a volume (package, basket, bucket, pack, cans, bottles, etc.):
корзина клубники (a basket of strawberries), банка сметаны (a pot of sour cream), пакет кефира (a box of kefir), плитка шоколада (a chocolate bar), ложка мёда (a spoonful of honey), etc.
An indefinite number of conditionally issued volume of substance (a handful, a bunch, a pinch, etc.):
пучок укропа (a bunch of dill), щепотка соли (a pinch of salt), горсть песка (a handful of sand).
