Tayammum (Arabic: تيمم) is the Islamic act of dry Ritual purification using a purified sand or dust, which may be performed in place of ritual washing (wudu or ghusl) if no clean water is readily available or if one is suffering from moisture-induced skin inflammation or scaling.
Tayammum instead of Ghusl is allowed only if Ghusl is not possible e.g. no water, no enough time, health reason preventing Ghusl, etc.
Convenience alone is not a valid reason to leave Ghusl and do Tayammum in stead of it. Ghusl remains obligatory because it is possible even with little difficulty, and in such case Tayammum will be invalid.
When Ghusl is not possible, Tayammum must be performed to perform the obligatory Prayer within its time. Tayammum can be performed on any material of earth e.g. dry soil. Sand, stones, dust, etc. Wudhu does not suffice when Ghusl is not possible, but Tayammum is the only alternative.
Grass itself is not part of the earth hence Tayammum on grass is invalid, but it the grass is covered by soil or clay, Tayammum will be valid on soil or clay or sand etc being part of the earth.
Tayammum is to be done, but it is good to do both Tayammum and Ghusl as precaution.
Yes it is allowed to move the palms during Tayammum right and left over the forehead.
It is also allowed to move the palms down till the upper part of the nose.
Tayammum is the obligatory alternative instead of ghusl or wudhu in certain situations, like:
1. When there is not sufficient water for either ghusl or wudhu.
2. When there is sufficient water, but there may be a barrier to getting, or using it.
3. When there may be a possibility that using water for ghusl or wudhu might deplete your supply of water, so much so that there may not be sufficient quantity of water to quench one’s thirst, including members of your family, relatives, community and even the livestock.
4. When there is not enough time to perform ghusl and pray an obligatory prayer in full and within its time.
5. When obtaining water could lead to putting oneself in an untenable situation.
6. When water has changed in both colour and property to the extent that you find yourself in real in a state of revulsion when using it.
7. When your responsibility is to use the limited quantity of water to remove the najasah from the mosque.
8. When you fear you become sick if you use water for ghusl or wudhu, or if using water could worsen your ill health. However, this should be the case when the matter does not come under the subject of wiping (mas’h) over a jabirah (lit. splint: a generic name used for any form of dressing any part of the body after injury or illness), especially when there is a risk of worsening the condition of the injury or ailment if the dressing is removed, and the part of the body is not among that which is covered by the ruling concerning.
“Tayammum is obligatory to be performed instead of wodu’ or ghusl in seven circumstances:
1. Unavailability of water
2. Excused for not being able to reach water
3. Use of water constitutes harm
4. Fear of thirst
5. Scarcity of water for washing
6. Non-permissibility of use of water
7. Time being too short”
May Allah grant you success