(Reuters) - India introduced a new national Goods and Services Tax (GST) in July last year, aiming to unify its $2.6 trillion economy and 1.3 billion people into a single market for the first time.

After 14 months, small businesses are struggling to keep up with over 200 tweaks in the tax system.

Below is a description of the tax and some of the major changes:

WHAT IS IT

The GST replaced more than a dozen central and state tariffs, duties and fees levied - often at different rates across India’s 29 states - into a single tax.

All goods and services have been put into one of five tax brackets ranging from 0 percent on food products to 28 percent and above on luxury goods such as sports utility vehicles and “sin” goods like cigarettes. Other goods and services are taxed at 5, 12 and 18 percent.

CHANGES IN TAX RATES

* JULY 2017: Government raises GST on cigarette makers within a month from launch, causing a dip in share prices of tobacco companies.

* SEPT 2017: GST on medium-to-large cars and sports utility vehicles raised by up to 50 percent, which led to an increase in retail prices of up to 7 percent.

* OCT 2017: Government reduces tax rates on 40 goods and services, including food and textiles, in response to public protests.

* NOV 2017: Government lowers tax rates to 18 percent from 28 percent on 178 items and cuts penalties for late filing of tax returns.

* MAY 2018: Government takes over GSTN - the IT company handling tax filing data.

* AUG 2018: Government cuts GST rates on 88 consumer items

CHANGES IN TAX RULES

* JULY 2017: Government first extends tax return filing dates after software glitches. Authorities have been forced to extend the deadline several times since.

* OCT 2017: Tax deposit rules relaxed for small firms with an annual turnover of less than 15 million rupees.

* APRIL 2018: It becomes mandatory to inform the taxman on planned inter-state transport of goods.

* AUG 2018: Government seeks parliament approval for 46 amendments. bit.ly/2MWFXDc

OTHER COMMON COMPLAINTS:

* State authorities have different interpretations of which category some items fall into, resulting in different tax rates for the same items in some cases. For example some states said solar plant installation work should be taxed at 18 percent while others said it fell under the 5 percent category.

* Small differences between items can lead to big differences in tax rates: coffee and tea are taxed at 5 percent but coffee beans and green tea leaves face no tax.

* The government has yet to issue its standard form for tax return applications for the fiscal year that ended in March.

* Software glitches mean that some invoices don’t match and many manufacturers and exporters say large sums of money are stuck with tax authorities causing them cash flow problems.

* Petroleum and alcohol products remain outside GST and subject to higher taxes.